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.

Topeak makes a really great quick release crate, but it is proprietary to their rack system. It's even foldable.

It's also very expensive.

You could also put a collapsible crate on there, like a CleverCrate, those fold down flat and you can then bungee whatever you like on top of the flat folded crate, and unfold it and use it as a crate as needed.

I hope such a thing exists. It seems to me that it would fill a demand.

Long ago, I had a Pletscher rack with a spring clip. I used the clip to attach a milk crate. It wasn't super secure, but it was good enough.

Topeak has worked well for me. Not cheap but it works.

KLICKfix Rixen & Kaul Coupling Set GTA (for carrier adapter)

Not a crate, but what about the Wald folding pannier baskets? Wald 582 Rear Folding Pannier Basket. I've seen a fair number of bikes sporting these and they seem to work very well. I also rode a rental that had them and was pleased that they don't rattle at all.

The Walds do work well (the knockoff lookalikes do NOT, they fall apart under load, but the Walds are solid) but they are so solid that they end up being pretty heavy.

I had two of them on my commuter before realizing how much extra weight two Walds add, now I just use a Basil basket that hooks onto the rack rail, easy to take off and on and I can take it in and use it as a shopping basket as well. Toba makes similar baskets for less as well.

Years ago (1992) when just starting to commute I used long flat strips of Velcro to secure a plastic milk crate to the back rack. These were 4 inch thick and a couple feet long long. They were designed to hold fishing poles to boats. For the last ten years I've used Wald folding baskets.

Very nice, and I never heard of Velofred. What a great site.