When we bought our CC23 the previous owner had a layer of lightweight egg crate foam on top of the standard foam cushions. We found that alone didn't make enough of a difference so we added a layer of 2" memory foam and then kept the egg crate layer of foam on top of that. So we use: (1) the original cushion, (2) the memory foam on top of that, and (3) the egg crate foam on top of the memory foam. While it sounds tall it really isn't because the egg crate is so thin. We have found the egg crate allows more air circulation than the memory foam, while not adding much depth. Sleeping is now quite comfortable. We trimmed both layers of foam to fit the V-berth area, using the original cushions as a template. While cutting memory foam can be a trick, using a standard electric carving knife makes it a snap, like cutting through butter. Just mind where your fingers are.... When you cut the foam, remember to angle the cut and make it wider than the original cushion since the hull widens as it extends upward. By the way, we generally kept the foam in one big piece, only cutting out a small section to use to cover for the porta potty insert.
As for sheets, Mary bought a nice high thread count set of sheets and then cut them larger than the foam cushions but still using the general V shape. She then stitched the top and bottom sheets together at the foot so the top sheet couldn't come loose and pull out. By tucking in the bottom sheet under the foam cushion, and with the bottom section stitched and tucked in, we have a most comfortable sleeping environment in the "forward stateroom."
Rob