I bought an elastic one because it was only $5, and it holds shells just fine. However, it keeps creeping up towards the receiver when i shoot, so i have to adjust it frequently. Someone had a more expensive one (I think like $10) that also had a strap that ran across the butt. This should hold the piece in place and keep it from creeping.
Of course, if you plan to use it a ton, probably spend the extra money and get a receiver-mounted unit instead...