Elnut, another option is to purchase one of the soft sided rooftop carriers. They are made of a vinyl coated type canvas and are waterproof. Amazon has them from $40.00 to $50.00 bucks.

The big advantage with the soft side carries over the hard shell carriers is that you can fold the soft side carrier up to store it. Believe me thats a BIG plus (and they are cheap).

Might try wally world or Kmart as well. Use the Amazon price as a guide.