A mill is good but I use the real precision instrument: the angle grinder. The slot ends up about whatever width the cutoff wheel is. It doesn't need to be especially wide because you're only taking up a very small amount of diameter with the bolt. I make a little cylinder on the minilathe, drill and tap it to M6, TIG braze it on, and then cut the slit through the whole lot to join up with the hole. Because the cutoff wheel is quite thin I can then modify the slit a little bit with a dremel just to make it sure it hits the hole nice and centrally. Then I drill out one half of the cylinder with a 6mm drill bit so it's not threaded any more and the bolt goes in from that side.