I vote milkcrate- preferably one that you find. And attach it with 2- 8" long metal strips from any hardware store with bolts and wingnuts for easy removal. Then you can just toss in the backpack and off you go. Then if you pick up some groceries on the ride home you wear the back pack and put the groceries in the milkcrate- and mine fits two grocery bags side by side. and if you've got a fat butt like mine you may have to cut the milkcrate where it interferes with the gluteus maximus. Oh, yeah and get the beefiest, sturdiest rack you can get.