We designed our Dinoot M-Series M416 camping trailer tub kits to have the same 40.5″ width that a M416 trailer has to be used as replacement tubs. This also makes them the same width as a Harbor Freight 40.5″ x 48″ frame kit. The Harbor Freight frames are perfectly sized for building a stubby 4′ M-Series M416.
Here are a few ideas on how you could build one.
Using a Harbor Freight Haul-Master #62645 40.5" x 48" frame kit, I would first stretch the tongue with a piece of 2.5" square tube.
Add reinforcement plates where the tongue bolts to the cross-members is an important step.
Another possibility would be to make the tongue long enough for using a Harbor Freight receiver cargo basket as a front rack
The final piece is the tub kit. Just like a full length M-Series M416 tub kit; you could build it with or without a tailgate. We have done this for the 5' versions. We cut the side panels to the proper length for you before sending them.Here are the 6' and 5' version M-Series M416 trailer tubs.