I built an open utility trailer from scratch, but not an enclosed one. From what I'm gathering you're looking for something like the Featherlite here
http://www.fthr.com/trailers/style_t...lity&cat =rec ?
They use an aluminum frame with an aluminum sheet as the exterior skin. As mentioned pop rivets are used for the skin and some of the cross pieces in the frame. The main frame parts are TIG welded to the chassis for strength.
What is the frame that you already have? How large will this trailer be? What do you want to do about doors?
Now keep in mind too what the total weight will be (gross) of the trailer PLUS the stuff in it. In some states it will require it's own brakes if it's above a certain weight and may require a second axle (tandem).
If the trailer will be large for safety's sake get a sway control for the hitch, last thing you want is a cross wind catching the trailer and making it sway and thus making the tow vehicle sway (especially with smaller tow vehicles). They call it "the tail wagging the dog".
237 did have a small enclosed trailer (8' Haulmark) that a previous mentor (now deceased) had purchased. Once that trailer was filled with our stuff it was much heavier, will trailers you have to remember that the contents weigh more that the trailer itself. He had no trouble bringing the empty trailer home with his Aerostar but once filled the Aerostar wouldn't pull it. Add in wind resistence and it gets worse.