You may consider using a stainless steel or aluminum backup plate that is
machined to fit on the inside of the transom. I made such a backup
mechanism for attaching a water ski bar/hook on the back of my boat when
it was located rather close to the outdrive hole. 'Worked great. In my
situation, I used 5inch by 1.5 inch aluminum C-Channel and used an end mill
to tailor the fit with obstructing hardware.

b.