Great info on this site Sam.
This is what I did. I fabricated a 1/2″ thick, round, aluminum backing plate to attach the spare to the engine cover on my VW based TD. I used the stock VW 4 lug wheel to mark the holes. I drilled and tapped 2 of them and drilled the other 2 clean through. I used these last two holes to attach the backing plate to the engine cover and the two tapped (threaded) holes to attach the wheel using the OEM style VW lug bolts.The backing plate and spare tire sandwiched the fiberglass engine cover.
Another quick way to make one of these “backing plates” is to machine a VW brake drum until just the flat center that has the 4 lug bolt holes is left. I wanted the lighter weight. This is assuming you have an IRS donor and it has 4 lugs, not a “wide 5” swing axle style.
Be sure to use large diameter fender washers on anything you attach to the fiberglass.