Hi Rob,
Good idea.
As part of my Healey's Bose sound system, I had just redesigned the mounting of my Bose free air subwoofer mounted in my spare tire. Not having room for the subwoofer, I used the spare to provide a non-obvious housing for the Bose subwoofer. To provide sufficient amount of air flow, I mounted the speaker in a piece of 3/4" MDF that fits within center of the wheel and raises the wheel and tire 1" above the boot floor. This increase in height allows the free air speaker to perform properly but also increased the needed block height a little more than an inch. My initial idea was to create an 1/8" thick piece of sign aluminum to bolt into the original holes and attach the block at the upper end of the plate. The aluminum will need to be bent to conform to the angled couture or the boot wall but will hold the wheel tightly around the speaker mounting.
I like your idea because it would provide additional rigidity to the mount extension without requiring additional offset space required by block mounting nuts.
Glad you posted,
Ray(64BJ8P1)