Overstock.com offers 4"+ memory foam and quilted down zip up covers to put it in.
We bought a 4" thick California King and cut it with an electric knife to match the contour of our V-Birth cushions. You can just tuck the extra cover/bag under the memory foam. We have it all covered with Purple sheets to match the boat. Very Nice!!!!!
One downside is that memory foam is heavy and we added about 40lbs to the bow!
If I was to do this again I would bump up to 5 or 6" foam.