Polak, I've used a variety of waterproof bags like the PVC ones posted. In time, with moderate use, every one get a few leaks at folds, seams, or abrasion points.

You've probably already done this, but FWIW: I use GOOP or SHOE GOO to reinforce selected points, and to patch holes, pinholes, and abrasions. Ugly as sin.. but it works. The patch outlives the rest of the bag.