for an external overflow, I don't need a dremmel. I use my rotozip, it's faster. If you take the top trim off the tank, you can just cut the big U out of the glass. Then you silicone the trim back on and you've got a full top support.
To drill a hole, you need 2 bits. one is a ball, one is a cone. Both are diamond coated glass bits. I've spun lots of coating off bits by either going too fast or not enough water. but the rotozip is SO much faster. It's a tile cutting bit, so it's not the smoothest cut, but who cares when you're going to cover it with either a bulkhead or some silicone.
B