Countersunk 4 holes on the keyboard retainer, and started marking/drilling the IO ports out. Very carefully this time- cutting WAY inside the outlines- which were traced on the inside directly from the ports themselves. Not sure how I'm going to cut them out yet- maybe gentle (plunge?) milling on the Bridgeport, whenever I get time to use it.
The countersinking chattered like mad- the holes are unsupported underneath- but the screws won't stick out, so I'm calling it good enough.