A simple but effective technique to prevent a lot of swearing when the drill hits the windscreen glass sending a crack across at 5000kph(a true fact) is to slip a razor or safety blade between the seal and glass. The drill tip with then harmlessly hit the blade if you go to far and save you a lot of money and effort.

Slipping a blade under the windscreen seal. Withe the blade in place the Moke Windscreen is protected from the from the drill tip.