Why don't you just buy the Uncle Mikes 870 cap with the stud already installed in the cap?

http://www.uncle-mikes.com/products/remington870express.html

If you feel that you must drill a hole in the cap then you drill it slightly larger than the unthreaded part of the shaft and use the supplied nut on the inside of the cap with some loc-tite and a center punch to keep the nut from unscrewing from theswivel stud.