If your doing some electrical projects you for sure have various heat-shrink tubes laying around. Its primary usage is for wire insulation. I use it often for other tasks. Some of them I would like to share with you.
Take the tube a little bit wider than drill bit you will use. Cut it properly and put it on drill bit. While drilling look at the tube: as soon as it touches surface stop drilling. That way you will never drill through. (In my personal blog http://www.jumbleview.info/2016/04/not-for-insulation-only.html you can find some more rumination regrading this usage).