Follow-Up:
We tried various options, e.g. thick mattress pad, and foam egg-shell mattress. Best option we have kept is a regular sized 120 volt elect. inflatable mattress we bought from Wal-Mart. Blow it up partway, put on the sheets / covers and finish blowing it up. The sides of the mattress will push against the sides to hold the sheets/covers in place. A regular-sized inflatable mattress won't cover the total length of the Rialta mattress but using an inflatable mattress on the Rialta is at least comfortable.