Locate the touch hole in the proper location for the lock (I like the fence right even with the back of the plug) and, if the recess in the plug is not deep enough for that, drill the recess deeper until it is just at the back of the liner threads. An end mill (as opposed to a drill bit) would be nice as it would leave a nice square bottom. Held tight in a good vice and run slow, an end mill would work in a drill press since the recess is already well established as a guide.
