Iv tried a few bags and they all seem to look shit or way to expensive. Found a review on the net below and thought id give it a try.

Found this review here  http://www.mactalk.com.au/content/streamtrail-25l-waterproof-backpack-1923/ 

product: http://www.hardtofind.com.au/products/DryTank-II%252d25L.html

I don't like wearing backpacks so mounted it to the side of the bike with alot of zip ties and secured it in. Its a massive bag that can fit my laptop and spare cloths/shoes for work easy as. Tested it when it was pissing down and defently works, You can twist the top of it and fold it on its self just to make sure no water does get it. Even comes with a plug on the bottom to drain water if you do spill liquid in it.

Here's the pictures of it on the bike.