Lots of different brands out there that use a “powerband”. Goes underneath the entire mattress and should help keep it in place. Another option is ordering bed sheet fasteners from Amazon, pretty inexpensive and does the trick just takes slightly longer to make the bed initially.

Have a split king and I hate it. Pain in the butt to make, seems like less room because of the split in the middle, sheet sets are expensive, and it’s a “your side or mine” conversation which kind of kills the mood. You basically are trying to have marital time in your college dorm bed. (Twin XL). They are also waaaay more expensive. Wouldn’t do it again for any reason. If you do, the best sheets I have gotten for mine are from tempur. The band that helps keep the bottom sheet on makes them great and helps when it comes to making the bed. Sometimes during back to school season, I snag some cheap twin XL sheet sets to use for the bottom sheets and then either the coordinating king top sheet or just a plain color top sheet that can be purchased as a separate.