I bought some mag holders from "tactical kydex" on this forum and I love them. They are $10 a piece and he makes them for all Glock models. They are specifically designed to fit Glock mags and have adjustable retention.
http://www.glocktalk.com/forums/show....php?t=1267097
It's an older thread, but if you PM him, he should be able to make them for you.