Doug, my '72 L trunk insert doesn't have a hole either, but the steel nut is visible with a strong light inside the fiberglas. Careful drilling should expose the threaded nut. I never drilled it because for better balance in our car, I put the folding sidewall spare up front as it was in the prototypes, but with a hold-down system up there that IMHO it should have had from the beginning.
I mounted the spare upside-down in the trunk (with a tire cover) so I can carry some stuff inside the wheel's backside. Thus it takes a much shorter than stock hold-down bolt & the welding of a nut & washer to the underside of the front trunk floor. But doing that really requires a dropped battery as in the post-Ford-built cars. Always something....