BOBgear | BOB Bike Cargo Trailers – do more with your bicycle!
I have used one of these and it is great for 50-80 lb loads. The BOB looks to be something like what you have in mind. Build it and see how you like it. A trailer is not considered a part of the tow vehicle, car and truck trailers are liscenced separate from the tow vehicle.

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LennyHarp of Lenny's Bikes & Things
http://lennyh.com
The true value of a man is not judged by what he has, but rather by what he can do without.
"We are what we repeatedly do. Excellence, then, is not an action, but a habit." -- Aristotle