We replaced the cushions that served as seating for the dinette and sleeping about 2 years ago. The old ones were hard and had dips where people sat, etc.. There are places online that will custom make mattresses for you. We decided to check out a local company that makes mattresses, primarily for hotels but others as well.
They could have made us a coil spring mattress using 6" coils but after padding and the cover were added, it would have made the mattress 8" tall and that was too high given we have cabinets that overhang the foot of the beds and we felt there wouldn't have been the clearance for our (my) feet. So we went with foam.
They proposed a bottom layer of harder foam and a top layer of softer foam. What we liked about this place was that they mocked up different thicknesses of each and allowed us to lie on them to see which was more comfortable. That we could have never gotten online. We also had our choice of covers and chose the one that was 100% cotton.
I had to build platforms for each of the beds, since the original cushions had wood backs. I also bought slats from Ikea to give the surface more spring. If you buy the queen size bed slats they come in 2 sets, each 27" wide. The slats are spaced and stapled to webbing. I just cut the right length and stapled the webbing to the platforms.