Actually the Traxxas VXL 1/16 chassis is waterproof, so no need to do anything. If your nervous about the batteries, just wrap them in plastic wrap, and the plugs as well..... read this link..........
http://www.traxxas.com/products/elec...tro/intro7.htm
I wouldn't wrap with DUCT tape..for 1. It will make the batteries thicker and might be a tight fit..2. All that sticking duct tape residue...arrrgh, I wouldn't do it.