Lonny Grafman writes in about a tutorial on making your own bike trailer. The folks at the Appropedia wiki write:
It’s a lot of fun hauling cargo around on a bike. A bike trailer can make a bicycle more practical for transporting groceries, laundry, friends, etc. This is one of the least-expensive bike trailer designs out there, all that is needed to build a backpack-frame bike trailer is an old external-frame backpack, 18 feet of metal conduit, and some other hardware.
Make your own bike trailer from a frame pack – Link.