Actually I think something long and skinny works better like a bread bag. At work we have something like that but it is much heaver than a bread bag. I would put the bag on the inside of the shoe so you don't have to worry about it wearing through or if you have clipless pedals.

I don't suggest this to be cheap. I have had simmular problems with shoe covers as one of the above posters has and have found them to be next to useless.