There are several approaches. One is to simply build the new structure over the existing back end of the car, leaving most to all of the original structure in place under the boat tail. Advantage is the boat tail is solidly attached to the car, providing additional crush space in the event of a rear end collision, and you can extend the car's interior into the boat tail. Disadvantage is that it is a fairly permanent addition to your car and depending on how you design it, it could block access to the stock hatch or trunk.
Another approach is to install a Reese hitch, then build a boat tail structure supported by the hitch and stabilized by 2 additional attachment points at the upper corners. Advantage is that it can be removed from your car when not needed and can provide trunk storage space in the boat tail. Disadvantage is it blocks access to your hatch or trunk when installed.
A third approach is to buy a spare tailgate, build the boat tail structure onto this spare tailgate, then swap out the stock tailgate for the boat-tailed gate. Advantage is that, with some effort, it can be removed when not needed and it still allows access to your stock hatch or trunk when in place.
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