The rear portion of the body would need to be re engineered some to create strong mounting points for the hinges & latches that would resist flexing/weakening over time particularly if you want it to hold a tire and such. Not to mention, re-routing the wiring, adjusting the weather seals probably, redoing the rear latch system, and/or other considerations that you may run into.
Over all the cost of the fabrication, painting, and research alone would probably exceed the cost of a fully loaded CBI rear bumper which is north of $2k.
The above design concept vehicle is deceptively simple looking in terms of what needs to be done. I wouldn't be surprised if the vehicle's hinges are not very structurally designed as that was a design concept vehicle and not intended as a production vehicle and it probably would not last very long under regular use. But basically the design seems to utilize the FJ Cruiser hinge set-up with a modified 4th gen rear door and some re-worked sheet metal on both the door and body.