I have just recently started using my motorized bike to commute to work. I needed a place to stash tools, my lunch, and anything else I might need. I picked up a couple plastic ammo crates from a local surplus store and made them into a pair of nearly indestructible panniers. Since my commute is at 11pm I added a couple reflectors to help make me visible.
made this detatchable one sided rack mainly to hold a bag, but i think it will be sturdy enough for a light load
edit: forgot since i made the rack. the little u shaped piece under the front rack mount lets me put a small bike bag there without it falling back into the tirre or spokes
Made from some aluminum trash picked square tubing, a piece of random 3/8th plywood, a piece of plumbers tape(strap), and milk crate.
Holds the kids backpack, which is stuffed full, no problem.