I installed a ND tower on my 89 Supra . I threw away the 1/8'' aluminum backing plates that came with it and made some 3/4 '' plywood backers as large as I could fit under the upper deck area where the tower legs were located. Each plywood backer was coated with 1 layer of fiberglass mat and let dry then they were epoxied to the under deck . I then made larger 1/4'' aluminum plates that go over the wood backers to support the screws that mount the tower. 4 years no cracks.
The OE backer plate and the new larger plates
Tape off the deck to prevent scratches and allows easy lay out marking
Make sure you chamfer the holes to prevent gel cracks