Froli mattress system

While it is expensive, it does do what they say. Assembled from a bunch of snap together pieces, so you can make it fit pretty much anything. It keeps the mattress elevated about an inch, leaving much more air circulation than something like hypervent. It adds a slight box spring effect to the feel of the mattress, making it softer. More comfortable is subjective, but I think so. I've have the system on the boat for 10 years and not regretted it. It stopped the condensation we were getting there and made sleeping a dream :).

One downside beyond the cost: the design of the plastic pieces tends to catch on sheets etc when you tuck them in, a slight annoyance when you go to pull everything out for washing day. Just something to watch for.