I posted a YouTube video on how I did it. Hope this helps someone.
https://www.youtube.com/watch?v=XHldK-mgr3E
Chris
Hey, I heard my name; sure hope the idea(s) work...!!
Looks well thought out, and not unlike something I would do myself. If there was one (1) thing I'd recommend that you consider, it would be to extend the bolts through the trunk floor where your receiver is notched and bolted on.
The four (4) bolts** through the hitch and into the ends of the crossmember are
in shear, and the cantilevered load from the receiver will exacerbate this. You could get by with a plate on the inside of the trunk as thin as 1/8" and DO make sure to generously radius the four (4) corners, so as not to pierce the trunk's floor like an old style can opener! Look for some strong, high tensile strength bolts with shallow heads and put a circle of foam around them so they can't abrade the bottom of the fuel tank. After that (and the note below) I'd call it done, and done well.
** for the bolts into the xmbr ends, you should find some "grip length" bolts that have a solid shank the thickness of the 1-1/2" x 1/4" plates + the thickness of the plate with the captive nuts. You don't want a fully threaded bolt, as the threads will act as a milling cutter against the plates.
https://www.pegasusautoracing.com/productselection.asp?Product=AN7
Additionally, giving it more thought; running the bolts through small tubes in the square tube filler pieces will prevent those bolts from loosening in use. As long as there's an air-gap through the tube, it can compress and loosen the bolt's grip. If short sections of 1/2" inside diameter
black iron (gas) pipe were cut precise enough to fit tightly, they wouldn't even require being welded in.