Dekker

If you just want to add a hole (or two or more) just for aesthetic purposes, consider just having it lead to a false cavity in your cabinet. Maintain the required internal volume for the main cabinet by making it bigger by the amount of space the false cavity takes up. If you want it functional, the above are correct: the baffle thickness "is" your port. The purpose of any port is to (generally) increase the bass output, and it can only do this if the port is the correct diameter and length for the drivers in your cabinet. As BF mentioned, consult any number of speaker modeling software to determine these parameters. Having the incorrect parameters can (and probably will) result in time smearing, and an overall "muddy" sound.

I'm drooling for an RH750, but I really dig the functions of these heads. I envy you!