Interesting on plywood vs acrylic! Plywood would be MUCH easier to work with, too.
Speak to me of foam.
Fabric is my bailiwick. At a guess, I would probably use a sturdy two way stretch lycra, similar to bathing suits and yoga pants, because it would conform to the shape without wrinkles that would both be uncomfortable and cause friction points that would wear badly. I’m also thinking about linen or bamboo rayon for durability and to be cool to the touch.
Basically there are trade-offs, unsurprisingly. The obvious axis is durability vs comfort. I think there are a couple likely candidates.
It would be easy enough for me to post the fabric pattern on Spoonflower.com as a cut on the lines and assemble as per instructions sort of thing. They have a good selection of high quality fabrics and I love their print to order service, though I’m not sure how their printing process will stand up to heavy wear.
The idea in my head wouldn’t require any sewing, and if the base is plywood I would most likely use upholstery tacks, though prototypes might be built with a staple gun. all of which is idle speculation until I can get into my studio again. :-/