I think that Flex Foam is only slightly thicker than Bosal In-R-Form - pretty thin stuff
Depending upon the size of the bed's mattress, I would buy some of that 3/8" craft foam in the kids' section of Michael's or Hobby Lobby. It's pretty sturdy and you can layer it to make it as thick as you want.
If they're not big enough for the mattress size you want, you could glue them to one of those flexible cutting mats and cut them to the size you need.
Maybe glue on layers of batting if you want a soft surface? Cover with fabric and glue, then make a zippered mattress cover bag that could be washed.
The foams are nice for bags, because they add stability and body without much thickness, but they
are very flexible.
Good luck with your project - I know how important it is to make things for the grands.
