I've attached a bright orange file crate ($10 from the local Business Depot) to my rack: not a standard Canadian milk crate as the milk crate is smaller and I'd guess 3 times heavier/stronger. I don't need the extra strength, it seems.

It's attached with rubber tie down straps, not bungee cords. I can take the crate off if I'm not commuting.

I have quite a big back pack (day pack) and it fits quite well, it didn't fit so well in a real milk crate (smaller). I use this crate combo instead of paniers on this particular bike: if I need to stop at a store on the way home and don't have enough room in my bag for watever I buy, it can go in the crate, the bag goes on my back. The crate also allows me to carry three locks at the bottom, under the bag. The bag inside of the crate is not tied down in any way: quick in and out for the bag plus quick access to all the pockets. If I need to loose a layer, the extra layer can temporarily go into the crate as well. I also carry a garbage bag to cover the crate contents in case of rain.