Another comment on this back door issue --
The OP did not state what he intended to carry in the rear door storage, or how often he needed to access it. That could influence the choice of solution.
Here's another option. I recently adapted an ATV cargo bag for the roof area immediately above the back doors. We had a roof rack already in place, so it was easy to add this. I use a Telesteps 1400E telescoping ladder, lashed to our hitch platform, to access it (the ladder stores behind the driver's seat). It takes me about 3 minutes to put the Telesteps in place for easy access. My 6'1" husband can stand on top of our hitch's Yeti cooler and access it without needing the ladder.
This bag has about the same amount of storage volume - or more - as a door-mounted box, but was very low in cost, and the roof in that area can easily take a bit of weight (I used a cut piece of a rubber restaurant mat to put beneath it so it would drain rainwater and not scratch the roof - sourcing details
here).
This was a major storage space win for very little time, effort, and money. Bag fit:
Ladder access: