Hi beanbag,
I think your approach is a good one.
I don’t think you will find any suitable mattress materials that are cheaper than polyfoam. For temporary use I would use no less than 1.5 lb firm polyfoam and 1.8 or higher would be more durable. Most cities have foam shops where you can buy polyfoam but there are some online sources listed in post #4 here as well.
You can also read more about how natural rubber compares to synthetic rubber in terms of aging and durability in post #2 here (I forgot to link it in my last reply).
Phoenix