I would cut a piece out of an old bait cutting board, the thicker the better, silicon that on the transom with screws through it to the hull and fill the screw holes with epoxy resin. Then screw the transduced to the plastic. Then whatever you have done to compromise the hull is well sealed and covered from the elements.

Sounds like a plan. Is it ill advised to mount straight onto glass transom?

Hi,

I would like to add that if you go for a plastic or wood block it's nice to bring your countersunk bolts in from the back and fit the block with them on there so now you have studs and you can play with stuff and not be backing screws out and re sealing etc if you want to make changes.