Look at IKEA on-line at futon or love seat mattresses, one of those might work out for you. Some patio furniture pads may also work, but you would have to fasten them down with either velcro or anti slip material like that used under rugs.
We improve the seating comfort in our Scamp
13 by placing very thin sleeping pads like those used by backpackers, under the original foam pads. Good luck
Dave & Paula