Here's the punch that I've been using for a year:
Cartomizer Punch
Tighten screw until it touches the carto, then two full turns (720 degrees). If you want two holes, rotate the carto and repeat.
Test on an old, used, carto, to figure out the height you want to punch at -- different tanks have different thicknesses on the end caps.
Easy, consistent, I've actually done over 100 2-hole cartos on the same screw.
I posted what it's made of, don't actually have anything to do with the vendor. I do know that it really is being sold basically at cost of materials.