I found a fellow Minnesotan on the Make website today, and he's got a really nice-looking design to construct a trunk for rear racks from sheet metal and pop-rivets.

 Very clean and functional. I'm not sure what it weighs, but I am sure I don't care. The instruction set makes me think I'd like to take my first foray into sheet metal. Oh, and here's the obligatory "but does it hold a 6-pack?" photo:

Hell yeah, maybe even two...