Remflex DIY

The router is a good idea and another way to get clean cuts would be to sandwich between a couple sheets of tempered hard-board or plexiglass. Then cut the perimiter on a band saw and drill the holes. You will need to figure out how bolt the three together securely through waste areas, but you should get nice clean cuts.

Edit... A fine cut coping or jig saw may work better for the exhaust port holes if the hole saw tears, but if you can control the feed in a drill press it should be OK with the hole saw.