You can't solder to aluminum, the only options are tiny drill and tap but would be very difficult cause you cant go too deep and you need a tiny tap, also must be a bottoming tap, I wouldn't even attept it, if you can't use the stock screw holes then I'd run a wire all the way to a brass battery base for ground. Maybe could solder to the atty connector base but need a very hot iron, the aluminum body will act as a heatsink and suck the heat out, you could preheat the whole head with a heatgun then solder to the atty base could work well, or a black 20 ga run down in the corner you won't even see it