I had the same situation when I redid my 20'. I used aluminum counter facing which I ordered from McMaster-Carr [
http://www.mcmaster.com/] item # 1104A84. It looks good however I would suggest either doubling the number of hold down screws (it's pre drilled to accept oval head screws) or be sure that the transom cap is absolutely flush as mine deflected slightly when the motor was installed. Also, try TACO AFTERMARKET (800.653.8568). They sell Aluminum transom trim in several sizes as well as rub rail, hinges, bow rail etc.
Good luck with your project.