I installed mine right on the transom, but vibration and the motor mount screws will create their own havoc with the gelcoat, so I mounted a epoxy covered plywood plate on the outside, through bolted with three ss bolts through an aluminum plate on the inside of the transom, (port side) to take the abuse. The middle bolt is an eyebolt, to which I padlock the motor. The transom itself is strong enough on its own to handle up to the 8 hp rating for this boat, but you will want to protect the finish.
This link will take you to some pictures of my set up. They are the last two pictures on page 3, but feel free to browse this public site. Click on the thumbnail to see a larger image.
http://groups.msn.com/RogerConrad/shoeb ... ?Page=Last