i was looking for some other details when I spotted this picture about an installed watershield in one of Charles' (caspian65) blogs on a 65 K Convertible he played arouind with a few years back. For this item, the plenum and a couple of other paper based parts, I sprayed them with ScotchGuard to add a bit of preservative and to reduce any moisture problems. Some alternatives would be Thompson's Water Seal, clear flat spray paint (like Eastwood Diamond finish) lightly applied or something I've heard of but not used, hair spray. The idea is to waterproof something without making it look like it's been waterproofed.
Jim