seems to me that waterproof "outers" that are available nowadays fairly cheaply would work better, they sell them at MEC so must be readily available in the States too.
plus some bags inside for good measure if rain still could get in from the wheel side where the cover might not go over the whole thing.

http://www.mec.ca/Products/product_d...34374302693397

just seems like a messy job, and the stuff could crack or whatever, and might look godawful too...