I went about it a little differently. The Curt hitch you are referencing looks identical to the one that came on my Outback. it was only rated to carry 200# and one bike hanging off a carrier could easily exceed that when bouncing down the road so I would have never even considered hanging a cargo carrier in the receiver.
I took the factory hitch off and went to my local trailer/fab shop and had a 3/16" wall tubing brace welded to the trailer frame between the fore/aft channel iron. Then they welded some shot pieces of the same tubing to the bumper so that it was supported in four places. They also re-welded the existing welds on the bumper. The box sits on the bumper and is supported by braces that extend to the rear and is fastened down with self tapping screws.
The box I had fabricated to a specific size and it goes from the outside end of the bumper to almost the center so as to not interfere with my shore power connection. The box was also sized to sit directly under the trailer tail light without obstructing it. Cost for the welding reinforcement was around $300 and the box was $325 delivered to my front door. It easily carries a Craftsman tank type air compressor, screw type stabilizing jacks, and power cords but would carry a generator just as easily. A really nice mod is an electrical outlet that I tied into a rear wall outlet and installed in the box. One of the outlets is always hot and the other one has my compressor plugged into it so I can turn it on and off at the box without having to drag an extension cord to an exterior outlet. A 50' air hose allows me to air up the trailer tires and it reaches the truck also. Can you tell I have a lot of time on my hands? LOL
One nice thing about a custom box is that the depth on this one does not allow it to extend past the spare tire so no additional length is added to the trailer. Also, the contents of the box do not bounce as much as a receiver type carrier would.