I found that cutting the port hole with a jigsaw slightly undersize, then carefully lining up and gluing on the port worked great. Eliminated the need for a router template to get the hole right. After the veneer was applied, I then routed the hole using the port as a guide with a 1/4" flush trim bit. That left just a 1/4" round in the corners which looks good.
Be sure there's no dried glue on the interior surface of the port to spoil it's use as a guide.