So in post #4 above, I showed a pic of our originally configured hitch carrier with our gasoline jerry can and Yeti cooler, but it was always intended to be adaptable to the needs of specific trips.
We did this mod to replace the Yeti cooler with two cheap cargo boxes that we reinforced with steel strapping and angle pieces, and yes, there are six bolts directly through the metal frame of the hitch carrier, making the assembly more solid than it appears.
This approach was partly motivated by cost - the commercial rear door cargo box options are beautiful but spendy and more importantly, they are too heavy for our rig whereas these aluminum versions are only 15 pounds each (we need to keep GVWR in mind). And also, I don’t want to store a lot of expensive stuff in these, so why go high-end? There are so many other ways that I would prefer our Sprinter to eat my wallet. The bottom Vevor box will carry the bush dunny, the dunny tent, a tabletop BBQ (sooty/greasy) plus other odds and ends. The top one will carry a 5-gallon bucket, water filter, snake boots, wet weather clothing, tarps, etc. I don’t need to spend almost two grand just to externalize that kind of simple stuff. We used mostly surplus hardware on this, so our total project cost was about $300.
Image collage below followed by a cranky 37-second video which will play even if it previews the gray screen of death: