As I stated before, I decided to use screws instead of riveting the sub-panel covers on. It just makes the panel a little more modular in nature. Sometimes the key to working on these smaller airplanes with tight quarters is to make it as maintenance friendly as possible.
Anyway, I was able to easily cut the hole in the sub-panel after marking it carefully to make sure the plenum on the back side had enough clearance to thread onto the vent nozzle. The vent required a 2.75 inch hole and the plenum is 3.00 diameter. The cutout in the left sub panel was modified to fit the plenum with about 1/16 in clearance all around to tighten the plenum to the nozzle.
Fortunately the process was completed fairly easy by taking the sub-panel cover out into the shop and boring it to size with my mill.
The vent is from Stein's and is quality.....but not cheap! I bought the smaller vent for the rear seat pax due to the limited space for installing the vent.