i recently had to redo mine since avon decided to cut corners when fitting one.
all they done was bore the hole and flowcoat the inside of the hole. ofcourse that cracked eventually and moisture would get into the transom-i spotted it early on and fixed it with marine epoxy.
anyway i digress-if your going to do it there are a few kits out there if you prefer-
i personally would bore the hole, glass it up properly then flowcoat accordingly then fit this thing with sika-or glass in the entire assy if you can find something suitable.
whatever you do do not use normal silicon-pay the extra for sika from your local marina/chandlers.