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.
The “epicurian” brand is perfect for this, or their knockoffs. Its like 1/4" thick (6mm) composite wood. I have one that even fits in my laptop bag - way better than any designated “lap board” I’ve ever found.