I have a plywood platform for my bed and had used a 6" memory foam mattress with very good results for more than six months when I was full-timing a few years ago. It does not fold well, of course, but I don't need mine to do so.
For this trip, I bought the same thing but a different brand... both had been from Amazon and were just over $100 (free shipping as I use Amazon Prime). They come all suctioned down and rolled in a narrow box. You slit open the tight plastic bag inside and it all expands nicely.
A tip for cutting the memory foam mattresses down to size: use an electric knife if you have one or can borrow one. It goes through like a hot knife through butter if the blade is as long as the mattress is thick.