in San Diego
used to carry some stuff which worked well, not sure if they still do.
It looked like 1"/25mm thick 3M (green) abrasive scouring pads. So that the majority of it was open air space, & it's structure was just a lattice matrix of plastic fibers. So your mattress was resting on a piece of 3-dimensional mesh.
It'd likely be worth contacting them, to see if they still carry it, & if they have a POC for it.
Also, go to the Searunner
section of the multihull
forum here, & ask someone to post some pics of the structure of the bunks on a 31'er. It's a KISS design, based loosely on the gridded hole pattern setup, which does an extraordinary job of ventillating things.
I could sleep on mine in 40 degree weather
, sans heat, & have nary a condensation
The one structure that I've seen, which works similarly to Searunner
bunks, is a Futon frame. Where you have the mattress on raised slats, to allow air to flow through & underneath of the mattress. And the slats are rasied off of the floor, on a frame structure by several inches, in order to allow for better air flow underneath.
Again, a KISS setup, which works quite well. And one, which, on a smaller scale, could be adapted to boats.
Sadly, explaining it via text would likely take a full page. Thus the pics idea, above.