I have a rear rack now and I want a crate that can go on and go off with a minimum of fuss. I don't want to ride with a crate around all the time because I usually put other stuff on the rack such as my u-lock, which is secured by bungee cord.

Any crate that has clips or is designed to go on with a minimum of fuss?

Alternatively, any recommendations for a small, rectangular crate? Something that won't significantly increase the width of my bike? Another reason I don't want to strap on a milk crate is because that would make my bike significantly wider at the rear and given the crowded bike parking facilities here ... I'd just rather not deal with it.