You might try this site for plans for a BOB-like trailer -- http://www.biketrailers.20m.com/MAIN.htm

His is welded which is certainly neatest, but I'm guessing that some variations would allow a bolted contruction. I think his plan uses the BOB QR skewer for the hitch. You might also be able to just bolt the hitch to the seat and chainstays and leave it in place if you use it a lot.

You might also check out the Kool Mule trailer by Kool Stop. You could hang paper bags off of it in place of panniers for a very successful hauling method. I seriously considered building something like this myself using conduit and some flat steel for braces. Since you could ultimately create a triangular truss beam with a wheel at each end it should be very strong. I hope that helps.

Good luck with your project.