What kind of skills do you have? Welding? Steel? Aluminium? Timber deck or metal? Flat tray or with sides and headboard? What sort of tools do you have available? I assemble and install trays at work every day. Ours are aluminium and made from proprietary extrusions, but the build principles stand true. Basic construction consists of two longitudinal rai made of C section with the deck laid across. There is a perimeter frame for strength and to attach rope rails and hinges. We use mounting rails made from 50x50x5 angle running across the chassis with slotted holes at each end so we can centre it on the vehicle, bolted to the chassis mounted with high tensile M14 bolts and large washers. These have uprights made of the same material, with two holes in each and the longitudinal rails on the tray slide in between them and are bolted with grade 8.8 M12 bolts, 2 per upright. Usually there are 3 cross rails except for some Navaras that only have 2 and single cab generally has 4. Assuming you can do basic steel fabrication and have access to a welder grinder, and basic hand tools, I would do the following: Determine the size you need...our standard for a dual cab is 1800 long and either 1800 or 1900 wide. Width will depend on your wheel track, the most common I see seems to be about 1830 from outside of tyre to outside of tyre...you can use flared guards, or make the tray side enough to cover. Personally, I'd go with 1800x1900 most of the time. Assuming you have a reasonably flat concrete slab to fabricate on, I would do the following: Weld a perimeter frame the length and width of the tray out of 75x75x3 angle. Mitre the corners, and fabricate it upside down, angle up. Tack each corner and take cross corner measurements, if it's out of square clamp it square and tack some more. If it is still not square don't be afraid to cut tacks, reclamp and retack... get it right as this is the foundation for the tray. Once it's square, weld about 20mm of each corner and recheck...if it doesn't measure square and look good when you've finished welding this frame, it will never look good on the car. Take the time to get it right. Once you have that frame, weld some cross braces every 150-200mm from side to side...I would use 25x25x2.5 or similar, fully welded to the perimeter frame. When the tray is flipped upright, the angle iron on the frame will point down but the cross braces will point up. You can then cut some 1.6 checker plate to fit perfectly inside the perimeter frame and sit on the cross braces. Tack each corner, then weld to the perimeter frame. You can fully weld, but doing a 50mm weld with a 50mm gap will be plenty strong enough. Flip the tray upside down again, and stitch weld each cross brace to the underside of the checker sheet using at least 4 50mm stitches on each one. Again, you can fully weld if you wish but it's overkill. You can now weld rope rails, I would use about a 30-40mm round tube, 2.5 wall thickness, and 50x3 flat bar for the stand off brackets. Space the rope rail about 50mm off the perimeter frame and fit the stand off brackets every 3-400mm. I would just cut a piece of the flat bar and weld to the inside of the perimeter frame leaving 50mm protruding, centre the tube along them and weld both sides. Use a piece of angle at the front and rear to cap the rope rail off and at the rear use a longer one to act as one end of the tail light protector. Now fit the longitudinal rails. 100x50x5 C section would be my pick. C facing inwards, rails spaced about 1200 apart outside to outside. Make damn sure they are centred on the tray and parallel...tack, check stitch a couple of welds, check again, weld. I would weld these to every cross brace, both sides of the rail. Tail light protectors can be as simple as angle iron coming down on either side and a bar welded between them, or can get as fancy as you like. You'll need brackets for the tail lights, ours are a simple piece of sheet metal with a fold at the top, screwed to the cross brace of the tray... that works fine. Again, you can get as fancy as you like. Number plate bracket likewise. We set the number plate off to one side as most dual cabs have the spare tyre crank handle hole in the centre and we don't want to block it. Tray is now flipped upright again and you fabricate the headboard if you're using one, best to get a metal fab shop to bend some 2 inch or 3 inch tube for the perimeter, then weld that onto the angle iron frame of the tray with the forward edge of the tube flush with the front of the tray. I would then notch some 50x50 or 75x75 square tube 1.6 wall to fit in between, set the height where you want it, weld to the tube, then a sheet of checker plate from it to the tray floor. If you want sides, easiest way is a frame of 25x25x1.6 square, with 2 cross pieces ladder style at the points where the hinges will go, then weld a sheet of 1.6 checker on the outside and weld on the inside too, especially where the hinge braces are, then buy off the shelf trailer tailgate hinges and weld them on. Use a straight edge to tack one side of both hinges in line, slip the hinges together, clamp the side board in place and tack the other side of each hinge. Make sure they operate smoothly before fully welding. Latches are available off the shelf too...I would use the over centre hook and eye style. Make sure to build the rear board 50mm narrower than the tray and use an upright piece of 25x25 angle at each end so it sits over the side boards and locks into place. Paint it and you're done... Now for installation... Set each cross rail on the chassis, centre the front and bolt it down, centre the rear and bolt it down, use a straight edge to line the middle one up with the others and bolt it down. Sit the tray in between the uprights on your rails, line it up fore and aft, making sure you've got at least 50mm clearance between the cab and tray. Now the most important thing... the height of the uprights will vary from car to car, but usually the same length works for all 3 and 150mm is usually enough... spacing the bottom of the tray rails 50mm above the chassis. Some vehicles have one pair of mounts lower than the others and some have more hump in the chassis or shocks sticking up... you'll need to measure up first. You want the pitch of the tray to be level with the cab and a little bit forwards... Find a straight line you can measure, usually the pinch weld on the under side of the sill is good. Measure from the floor to this point at the front and rear of the cab, and note the difference. Measure the distance between the two points you measured and note that too. Measure from the ground to the front of the tray, go back by the same distance as the points on the cab that you measured, and check the height of the tray there too. The difference should be about 10mm more on the tray than on the cab...in other words, the tail of the tray should be 10mm higher than the front, compared to the cab. If you make it dead level, it looks OK empty, but with a load on it looks like it's dragging its arse a bit. If it is lower in the rear, it looks like crap all the time. More than 20mm higher in the back looks silly too. Remember this is compared to the cab, often empty the cab can have a 40-50mm rake which would mean the tray needs 50-60mm... heavy springs in the rear can make it sit higher still. Use spacers to pack the tray so it sits how you want it...we always use 5mm packers on the front rail, and as many as needed on the rear. We also have a big rack of rails off different heights to choose from so we get it close from the start When it's sitting how it should, drill holes in the tray rails and bolt to the uprights. You'll need mud guards, you can buy generic trailer ones or make your own. You'll need mud flaps and you'll need to make a bracket to hold your fuel filler... but this is all ready enough if you have the fab skills to get this far. This will give you a tray that's fairly strong and not too heavy. You can use heavier material for a stronger tray, but the weight goes up... you can use lighter material if you're only going to mount a spare tyre bracket and a tool box on it or the tray is for show... There are many possible variations you could use timber decking, make it from ally if you have the tools and skills, do a short comp style tray with no sides, the possibilities are endless...