On that note, has anyone built their own memory foam mattress?
I did. It was quite a few years ago and it held up better than I expected.
I bought everything online.
I bought a generic foam queen mattress (but not the cheapest I could find). It came tightly wrapped in a tube shape. When I cut the wrapping, it grew and grew - very much like a life-raft that gets accidentally inflated on a sit-com.
I bought a 3" memory foam topper - but I only bought a twin size.
I bought some 3 inch upholstery foam.
I bought a mattress cover for a 9" mattress.
I laid the memory foam topper across the mattress sideways and cut the 'overhang' off with a serrated knife.
I put the memory foam where my shoulders to hips will be.
I cut the upholstery foam to fit/cover the mattress where the memory foam didn't.
I put the mattress cover over it all to hold it together.
I flip or rotate the mattress and/or the topper every year or so.