I traveled to Hainan,China to go on a biking trip and needed some panniers. Didn't want to spend a lot of money so I went to a local market and bought two backpacks ($7 each - should be the kind that also have a strap that goes around the waist). I simply pulled the straps out of their buckles and put them in the respective buckles of the other backpack. Then I draped the two backpacks over my tent on my fender rack. Secured the bags to the side of the rack by using the waist-belts.
I could put the packs on and take them off the bike in about 30 seconds and I had no problems with them interfering with my biking. I secured everything to the bike with one long bungee cord.
First photo shows how I used them for 600 kilometer trip around the island of Hainan, China.
Second photo shows the two bags hooked together.
Third photo shows the waist strap is wrapped around the bike rack spar and then cinched tight so that the bag will not move around or get caught in the spokes.
Video of my trip around Hainan: http://www.youtube.com/watch?v=Qin6_01vf0w