Being new here, How wet does it get under the seats on each side of the cooler?

I have some scrap foam board about 1 1/2 thick (used on stucko homes) thinking maybe some type of square box around the cooler. it seems that on my model one could side the pieces in from the rear ballast and make the box and use metal tape to seal. have a piece of the black composite stuff on the ends to prevent chips from stuff in and out of the compartments on each side.

What is the boards on each side of the engine made of? Where can it be brought?

CD
2002 OB LSV