We've made custom cushions from foam we got from https://www.thefoamfactory.com/ https://www.thefoamfactory.com/ (no affiliation). They apparently do have "matt

We've made custom cushions from foam we got from https://www.thefoamfactory.com/ https://www.thefoamfactory.com/ (no affiliation). They apparently do have "mattresses", but also just stock a bunch of foam that they can ship to you and you can cut to dimension.