I will second that suggestion to get a futon mattress. The one I use is a firm, thin enough to roll up (4 layer) all cotton filled futon (no foam, wool, latex or polyester fillings in mine) that I have simply placed on top of my plush pillow top firm spring mattress. I got mine at Ikea, I've had it for 18 years now and it's as comfortable now as ever. If I had to put a futon directly onto a hard, inflexible surface with no give to it though, I would probably want to use an 8 to 12 layer futon instead but that would be heavier.
You can get futons in various different sizes and with other fillings as well and in layers up to 12 layers that I know of, possibly more layers if you are having one custom made. I find the futons with all cotton layers as filling are the heaviest futons but also they're the most traditional and most comfortable, breathable for me.
You can see some pictures of some of the types of layers plus description of futon materials and sizes of futons at this BunkBedsCanada site here:
https://bunkbedscanada.com/futons/futon-mattress.html and also you should look online for other futon suppliers that may have what is most suitable for you.
Here are some pictures of other types and brands of futons and several other manufacturers/suppliers for you to check out:
https://www.google.ca/search?q=futon...w=1338&bih=662
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