This us the mesh I was talking about. We used in our RV with a big heavy foam hybrid mattress and never had any issues with molding. https://www.mattressinsider.com/mattress-condensation-prevention.html
I have a tipi style floor bed. I would recommend a pool noodle under the sheet. It’s really minimised the amount of times he’s fallen out x
Generally just getting slats to go under is enough. They also make a mesh sort of liner that is often used for RV beds. That would be perfect too if wanting to put the mattress directly on the floor. Or the ikea bed that can be used on the floor or slightly raised or as a bunk.