Square Style Trailer

32 inch x 34 inch cargo trailer.
Pictured with some sample cargo.
Cargo area is about 20 x 34 (inches).
It's made from 1 inch square, steel 14 gauge tubing. (14 gauge means the wall thickness is .083 inches, http://www.engineeringtoolbox.com/gauge-sheet-d_915.html) This is way more metal than needed for bike trailer purposes so it's on the heavy side, probably about 30 lbs, but it will carry 100 lbs and be square for decades. Also, if you are comparing with other trailers out there, you will find this particular combo of weight and capacity is not odd.
Link to biketrailershop.com, showing a trailer for about $180 that weighs 25lbs and is rated for 66lbs.

The wooden platform is some lightweight paneling. Not specifically designed for this application but it seems to do the trick. It would break if you slammed something heavy on it. Any 20 x 34 piece of wood or sheet aluminum would be good candidates for the cargo floor if it needs to handle heavy slamming. I can install it for you.

Tires: Sunlite 20x 1.75 Street tires, 40 psi.
Tubes: Pyramid 20x 1.75 schrader valve.
Wheels: Wheelmaster 20x 1.75, steel, bolt-on, 5/16" axle, 36 spokes.

Hitch: Burley standard forged hitch, with flex connector and safety strap kit. The heart of the hitch contraption is the forged aluminum piece (silver piece in pic above) that bolts onto the bike frame at the axle on the non-drive side of the rear wheel. Any common bike will work; quick release, bolt-on, horizontal or vertical dropouts. But if you have a brand new bike with disc brakes or some kind of fancy drop outs, this hitch will not work.