You've already gotten some great advise, another idea is to use the
same material as RV manufacturers put underneath their plywood floors.
It is a (generally) black, woven fabric that allows water vapor to
pass, but stops the liquid. This allows the plywood to dry out (really
You should be able to buy this material from your local RV store or
Camping World (chain camping supply store)
I couldn't find it on-line (in a 30 second search), but I know the
stuff exists and you can buy it, because I have bought and used it