sealline makes little zip/velcro pouches that attach to bags with a loop. they are rated as 'splash proof' on their website but I have been through many downpours without having any water leakage in mine (I have the zipper one)

http://cascadedesigns.com/sealline/u...pocket/product

and

http://cascadedesigns.com/sealline/u...c-case/product