I used a rubber based paint that is used on pools. It's used for fresh and saltwater pools. Goes on very thick and helps with vibrations as well. I got it from Home Depot, had to order it on line.
I don't know why I didn't notice this before, but if your just using it for top off water honestly its probably going to be cheaper to just use a small glass tank unless you have the plywood laying around extra already as I don't think you use the cheap osb plywood.... atleast the ones I have seen don't.
I didn't have room for a small 10 gallon tank, but I bought one for $15 and sanded the black plastic frame off the sides and it fits perfectly in the space I had.