That is cool.

I use a Burley Flatbed Utility Trailer almost daily. I'm sure you'll get a lot of use out of your trailer. A very handy thing to have.

I considered a box like you have, and still may do it one day. The way I considered attaching the box to the trailer is to bolt a thin plywood bed to the trailer frame, then bolt the box to the bed through the bottom.

You might be able to get just a hitch from Burley, wouldn't hurt to ask.
http://www.burley.com/default.html