If you search your local CL for motorcycle trailer, especially around bike week, you'll find very similar trailers. Luggage trailers is another name for them.
Harbor Freight sells a cheap 8" wheel 40x48" trailer frame. They also sell a 12" wheel version for little more. Or take a small pwc trailer. Add a roof top cargo carrier and you have it. Cargo carriers are readily available in a variety of shape and style. Sears still sells their boxy carrier. These will sell along those $300-450 price range.
Some people become very creative as well. On Kansas City's CL, few months back, a mc trailer was listed. Made from an old single door refrigerator.
That was sold rather quickly, so another one popped up as well.
Both were asking for $650.
Here was my idea of a Jeep luggage trailer.
But the top needed new hinges, cracks repaired, and more work than I wanted to put into it. So I sold the carrier and bought a large yard dump trailer for the luggage trailer. Its going to be similar to a military M416 1/4ton trailer in layout and color.
Frame will be camo green and tub will be camo beige.
Got the idea for the Jeep luggage trailer from this