Not a fold-up, but we bought a closed cell foam 8 inch mattress from Costco.
I am sure you could cut it and make it fit.
We got the full size queen, and just let it compress when the slide is in. Now my toes don't hang out the end for the cat to play with!
Happy Glamping, Norman & Elna. 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back. Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!