Let me start with, I'm repeating some of the things others said and I want to thank others for the things they said.
For the second time in 3 decades I recently shut the tailgate of my car on my thermarest mattress. It is 20 inches wide, 72 inches long and about 3/4 to 1 inch thick and has 2 patches on the bottom edge. It is marketed as self-inflating but I add more air to support my bulk, especially at the hips.
When I flattened it I hastily bought a 2 inch foam camping pad, 24 inches wide, from a hardware store. It's just regular foam, not the dense foam some camping pads are made from. It wasn't enough, but it was much better than plywood. I patched the other in a few days, after I returned home.
I now have both of them zipped together inside a sleeping bag liner. Among other things, this gives me bedding on my mattress, something I had wanted. So, right now I have 2 mattresses of different widths inside a cover that is a bit wider still. Aside from that I think I have something I like very much.
The next plan is to cut the foam to the same width as the thermarest and have the sleeping bag liner reduced to the same size. The experimentation will continue from there. Which one should be on the top? Does it change between Summer and Winter? Thanks to those who suggested emergency or more durable reflective blankets. I'll see if they are better in the middle or on the bottom. Should they reflect heat up or cold down in the Winter? Should they be the other way in the Summer?
For many years I've found the 20 inch mattress to by sufficiently wide. Its also all the room there is between the remaining rear seat on one side and the wheel well on the other side. Until recently I had relied primarily on sleeping bags and similar products as bedding on my 20 inch mattress. Recently I started buying my camping bedding from the Drapery section of the Thrift store. I didn't notice until I got home, that all of my new 4 x 8 foot sheets and blankets started life in an Ikea store.
Thanks again to those who gave me some good ideas.
MG