Haven't you heard? ScotchGard eats the upper atmosphere. It isn't *that* waterproof, either...

Looks like it's in the same price range as (or cheaper than) "goofy bag designed for bicycle rack." Thanks for the pointer, it's useful to see how the motorcycle crowd deals with the problem. I'd generally trust such a thing to be a bit more industrial-strength than some of the bike stuff I've found.