This is a follow up to my comment on a foam topper. We ended up finding a 3 inch thick memory foam topper with zip-on cover, queen size, at Costco for about $160. We contacted our upholsterer and he agreed to cut it to fit the v-berth and to re-cut the cover for us. It was an excellent decision and a comfortable addition to our main sleeping berth. He completed it quickly over a weekend and didn't even charge us for the cutting and cover modifications, so we got a little more mileage out of the upholstery job.
Nice to see this thread pop up again. Good luck on your upholstery projects!
Craig