My dc pai 2k had to have them done an the size I used was 6mm set screw an a 6mm tapper. Just re threaded the hole an the screw fit righ in snug an fit.
I would go the 3-3.5cf. I have a pair an my box is about 7.16cf after displacement an all an hey get down pretty good. My box is tuned to 35hrtz an I can play down to 25hrtz.