Just another vote for consideration of these. Not a "rocket science" kind of solution, but not expensive and they work pretty well for me.I use these to stuff clothes or sleeping bags in. They are fairly durable and the zipper holds up quite well. I have done several trips with them exposed to the elements in the back of my truck without a problem. They make a couple smaller sizes too. I work for SC Johnson and get them on the cheap too.
http://www.ziploc.com/Products/Pages/BigBags.aspx?SizeName=XXL