Or you could get a standard pannier and one (or two) of those folding rear baskets, and drop a backpack into it. You still have the other side for a standard pannier if you choose. Plus, having the load low and behind you will be more stable.

There used to be a company called Cobbworks that made panniers out of five-gallon square buckets with lids. They were at http://www.cobbworks.com , but the site is down now. There's a guy here in Madison who makes 'em out of cat litter buckets and the like, but AFAIK it's just for friends. You might wanna try fabricating something similar.