If putting Futon on top was okay, how about try finding bed slats with shorter pitch? I feel like this could do the job if you had matching bed frame. (Or even buy two of them with pitch wider enough to allow other slats to fit in between each other? - Maybe it's easier to get a board cut to the size though lol) I use something very similar to this one as a frame to put Futon on top, with the pitch of 0.6in, and you don't feel the gap at all. In summer, I put Goza mat on top of that for cool effect - that thing is Tatami mat but only for surface, so you might be interested in that. If you could find this configuration, there's close to no need to worry about molding and all, while sleeping on rather hard support.