I had a quick muck around with some of my trailers and I found the closest box trailer match that I have is DotM Deluxe Optimus Prime's trailer.

Granted it's small (Deluxe scaled trailer on a Voyager figure) and the connection is loose but it fits well enough for the truck to drag the trailer along. Energon Optimus Prime's trailer is actually a perfect 5mm post but alas the connector is far too short and cannot connect to Optimus Prime's hitch.