I've had the chain fall off a few times and was looking for some sort of chain-guide solution. Ended up just making one myself out of some thick steel wire salvaged from a collapsible laundry basket, old inner tube, and a heavy duty zip-tie. I bent the steel wire to the shape I wanted, wrapped the inner tube around the seat tube a few times and then slipped in the chain guide before wrapping a couple more times. Cinched up the whole assembly with the zip-tie. It's not a very adjustable setup so the wire has to be bent just so.