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Being Tourists by Car | The Voyages of s/v Silverheels III
I believe this couple installed this system (or the similar springy type competitor) and they have raved about both the comfort and the coolness of getting the mattress off the actual flat surface.
I have seen the springy stuff, the Froli snap-together stuff, and a sort of low slat-type system. All of them work on the idea that putting an inch of space and some non-rigid support between you, your mattress and the boat leads to a big increase in comfort and coolness.
The only downside is that you might have to both raise your bed space's fiddles and/or use your leecloths more often, because your body is going to be a bit higher. Not a huge issue for most.