It's almost sounding like a one wheeled one would be easier to build. I suppose if you had a couple of old bike frames you could use the rear triangles as the outsides of the trailer and if you lined up the dropouts right (assuming they're the same size, you could weld some sort of bed between them, and use that as wheel attachments.
__________________
Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"