I enjoyed the shopping bag panniers thread so I thought I'd share this. It's an ordinary drawstring bag, cost was zero since it was a gimme. I've been using it for a couple of months as you see it.
Two nails bent into a U, zip-tied to the bottom drawstrings, the plastic clasp and nylon strap is off some bag or another and attached to the top loop of the drawstring, and a velcro strip crudely zip-tied to the side. The cords with the nails go around the seat tube and hook into the grommet holes at the bottom, while the strap loops around the head tube. I spent about 20 minutes making this, with (as you can readily see) no crafting skills.
The beauty of it is that it takes about 15 seconds to put on or take off, without cluttering my road bike up with a rack. It's sufficient for my work clothes or very light shopping, and although I've carried a six pack in it I wouldn't really push it that far.
Sorry about the cruddy pics - I guess my cheapness extends to cameras also.