I use the milk crate since it holds the 12 packs nicely, but on a mountain bike, it put's a lot of weight high on the back wheel and makes handling dicey at slow speeds. You would be better off with saddle bags or as another poster mentioned, zip tying the milk crate to the side, just make sure it's secure. Sometimes I do both, saddle bags and milk crate. I have a hybrid bike with 700c wheels and on that, it's much more stable with the milk crate on top of the rear rack.