I have had a pair of those for a few years and occasionally use them as panniers, with no modifications. Think I got the pair on eBay for about $25, if my memory isn't failing me. The top two straps go on the top rails, and with my Jandd rack the bottom closure will fit on the bottom rail (I've got the Jandd Expedition). Haven't tried it on my other bikes/racks but I imagine it'll work there as well. I just loosely knot the shoulder straps together on top of the rack. The outer cover is pretty water resistant, but the thinner canvas on the back (facing the wheels) is not.

They're also the only bags I've used as panniers that I've had passers by comment on and ask where I got them - for whatever that's worth. Has happened a few times.