I used 15 3/8 for the outer diameter, 13 7/8 the inner. And I have some further thoughts about the subject. I don't like what seems to be the convention for driver mounting with the 8 bolts so close to the edge that t-nuts have a hard time grabbing the workpiece.Next cabinet I build is going to have the woofer and horn glued and bolted to the baffle. I stumbled across some adhesive that is sold with an undermount sink kit at Home Depot. Supposed to be aerospace type stuff. Or epoxy would do.
Then I would let in a door through the back panel with a wide rabbet to maximise contact surface, like watertight doors.Apply a gasket material and use bolts in t-nuts. Then have access without disturbing drivers and establishing even higher "Q".
This is great fun, isn't it? Enjoy!