Well, being a hands-on kind of a guy, I would just drill it myself - and indeed I have done just that. Make sure the diameter of the drill bit is the size of the screw's shaft, not the size of the threads or anything in between. I like to screw my holes just a tiny bit shy of the screw's penetrating length so that the tip of the screw and at least one row of thread gets a chance to really bite into the wood. Once I have determined that distance you can wrap a thin strip of masking or duct tape around the shaft of the drill bit to create a stopping point. Or, you can simply use a Sharpie and mark the bit and you'll see your stop line as the bit spins. If it were me I would put blue painter' tape on the back side of the ashtray so you can freely move it around on the body of the guitar without fear of marring the finish. I would then use a combination of just eyeballing the desired position (and then marking the holes with one color of Sharpie), and then double checking the relationship of those holes with the ashtray out of the way. If your measuring results in a slight difference in position, I would use a second color of Sharpie to mark those holes. Then put the ashtray back into this final position and just look at it to see how it strikes your eyes. Readjust as many times as needed. A drop of denatured alcohol on a Q-tip with remove any unwanted Sharpie marks. When you drill make sure your drill bit is plumb to your bass body so your holes will not be slanted. If you have a drill press, then use that, but be sure to pad the deck of the press and get help holding the bass into position while drilling. If you're worried about the bit slipping while drilling, then mask off the bass around the bridge and mark your holes right on the tape - might be tuff to get a good visual on it though with the tape in your way. Hope this helps and good luck!
