Personally, I would drill (if necessary) and tap for an M5 screw. Depending on the diameter of the BB, there may not be room for the expanded end of a pop rivet or for a nut on a bolt. But the shell should be thick enough for an M5 bolt to be tightened, hold the guide in place, and not extend inside the shell. A little grease on the threads would be good, as the BB bears the brunt of moisture and road debris. Besides, an M5 tap will be useful elsewhere on the bike, like fender and rack mount eyelets.