I used to bike everywhere and have made several of these and given them as gifts. The site I've linked to sells kits in addition to giving instruction, but you can find nearly everything you need at a hardware store.

The plastic buckets are harder to find and can be bought new online. If you're as cheap as I was in college though, you can get them for free(!) by asking around at local grocery stores for square buckets (ask them to hold them if they get any in). I had the best luck at cheese counters and found that feta, olive, and mayonnaise buckets are often square.

This is the only bucket photo I could find in my photo roll. I moved to Austin with my main bike bucket that I made from scratch and I'll try to share a picture soon.