Go to your local shop, grab a bottle, stick it in a random cage, then try to pull it out. If the bottle is easy to remove, try another cage until you find one that has a decent grip on the bottle. I usually buy whatever cheap metal cage is in stock at the local bike shop. If they're a little loose, you can always bend a metal cage to make the fit tighter. I've yet to have one rust, break, or drop a bottle...