Many things work to keep moisture out from under the mattress. Some use Froli system. Some use slats. Some use plastic lattice. There is Den-dry or similar, but air needs to get to it.
I have not tried them, but what is working for me, plus being comfy, are Therm-a-rest camp type, self-inflating mattresses. I think I got them on Amazon or REI. I got a 30 inch wide and a 24 inch, so they fit in my 58 inch space.
The dog probably didn't need one on his side, but I don't hit bottom anymore, and so far, it has kept the floor from sweating under the mattress. Plus I have a waterproof mattress cover, which may be overkill, but if dog gets in wet, it protects the mattress. At first, I just had the 24 inch mattress, but it was moving under there so I was sleeping at a tilt until I moved it back.
At first, I had too much air in it, which made it very difficult to get around, but after letting out some air, it's working well.
