Hi Mark,
PM me if you still need a bracket
The key to a successful repair is using a "strongback" (backing plate) when fixing to fibreglass. Bolts, Rivet-nuts, Screws, Pop rivets or just a nut and washer behind will eventually tear out of the tub under stress.
Consider the original oval backing plate, even though its 2" long and an inch wide with two bolts they eventually tear through the tub to give you the problems we're experiencing. I have very little faith in a glass patch up job, the tub is too thin and the pressure and stress too localised at the strut point to make a good, long lasting repair this way. Glass fibre repairs are never as good as the original where the resin set as a complete item, repairs always have shear points where old meets new.
Pressure= Force over Area
Increase the area (using a larger or second backing plate) and the pressure is reduced. Combine this with placing the strongback over a good undamaged part of the tub, rather than just a local glass repair around the original damage and you should have a long lasting solution.
Relying on a bolt or rivet-nut is going the other way as you are still focusing point loads. Granted 100 rivets would be fine, but not practical.
The Henninger bracket repair some use is a similar example of the physics but NOT good since it holds up the cover trims around the aperture (if placed under them), promotes water leakage (around the bolts/screws put in vertically) where water naturally sits or looks an eyesore if placed over the trim.