I just invested in a starter keg setup, and I made a batch of carbonator caps for beer transportation. Using this article as a starting point I drilled the requisite hole using standard twist drill bits. The problem with large bits and soft material is that they tend to suddenly “grab” and make large radial cuts. A 7/16″ hole punch works much better, and the valves have no problem fitting into the hole. I got my punches from Amazon for cheap.