The camping section in major big-box stores, Wally and Target both, often carry inexpensive heavy, nylon mesh reinforced, PVC, dry-bags. I got a couple of them a few years ago in the 16" diam x 24" size and they ran less than $15 each.
They also carried a smaller one about 8" diam x 16" in the same configuration and material. Given that typical dry-bags are roll-top, so any excess space doesn't flop around, I went big.
The larger dry-bag has attachment points that would allow it to be carried as a duffel or rough backpack if you rig some light line as straps. I figured that hurricane season can mean rain, rain, and more rain dry bags might come in handy.
Your typical coated nylon stuff-sack is water resistant, pretty much proof against light rain and spills, but for real protection, at a substantially increased burden in weight and expense, dry bags are the way to go. For the money these inexpensive units seem pretty good.
I will add that if the available dry bags are too large it is pretty easy to separate the bottom, cut off any excess off the body, and restitch and seal the bottom back in place. Mesh reinforced PVC holds stitching well and can be glued and sealed airtight with common materials from any good hardware store.
Edited to add the last paragraph.
Edited by Art_in_FL (09/18/10 12:40 AM)