I tried to google a bit, and found things, but most of them was either wacky, unconventional cases to see what is possible, or an assembly/review of a case showcasing the various machines used during the making, instead of design thoughts about what makes a case good and how can one design one themselves, but this may be because I didn't used the correct keywords during search. But all of them looks like professionals who do these things as a living, not an average Joe as myself.

I have two main questions about this thing, the first is the posts title:

Is it feasible or realistic to make a custom PC case at home as an average user?

And if yes, the second:

Where can I find design thoughts and guidance about making one? (Including pros and cons of different materials, shapes, sizes, fan and part locations, extension cables, and optimising airflow and noise)

Sadly I don't have a 3d printer, CNC machine or similar things, only maybe some screwdrivers, but I can ask around my friends to let me use theirs if they have one. Beyond this, I have infinite time to research and study from the internet.

Edited to answer the question what do I want to achieve:

I thought about a really simple design (which doesn't have to be unique or original), a box with good airflow and literally any shape (even concave if that's needed) which fits inside only the essential parts like motherboard, PSU, CPU cooler, and maybe a GPU if APUs won't become popular/cheap enough. Made from a strong, durable, and non-conducting material (which resistant to static electricity). Nothing else, no RGB, extension cards, fancy wires, decorations, screens, or even more storage than the M.2 SSD. Aesthetics doesn't matter, it doesn't need any "hide behind" parts and chambers for the cables, maybe only some rings to ziptie the cables to enable airflow. The "front" ports and power/start button can be anywhere on the case, on front, top, side, back, whereever it's blocking less airflow. One extra which would be useful is a handle on top to make it easier to carry it on the bus or such to LAN parties.

I built a PC recently and went with ATX with the Fractal Design Torrent Compact because ITX motherboards in Hungary are ridiculously expensive and rare, but I think a full ATX build is not my thing. It's a bit tedious to carry it around to friends for lan parties (not particularly the weight, but the volume of the case), and also the wasted empty space just hurts me inside.