I bought a piece of 1.5 inch stiff foam to try to fix a sag in a bed. It didn't work but having that thick piece of solid foam around has been a life saver.
Need something flat to put a laptop on? Throw it on the foam. Going to be doing something that requires you to be on your knees for a while? Get the foam!
It went from stupid purchase to something I'd gladly replace if it broke.
North Bayou, but only because I got a deal on them for cheap. Not sure if they are any better than any others, but they work as intended. Edit: these might be them: https://a.co/d/0MJRj7B