I read the threads and wandered the web. Talked to Marcy about her learnings. Today I bought a one inch thick gel foam topper from Mattress Insider (
www.mattressinsider.com). They cut to match the 78" x 78" bed platform in our 3124. Did not ask for backing, cotton pad, etc, which they can do if needed.
The gel foam is more open than the memory foam but still 4#/ft3 density. Supposed to be cooler. Should deliver early next week to my door for $152.
We have the '07 model year, which provides a 5" foam mattress in a reasonably durable cover. Not a bad bed, just a smidge hard on our hips. Hopefully this 1" gel foam will just take the hard edge of enough that we wake up without remarking about our sore hips.