Just a few seconds, start with a new bit, slow the drill down, lighten the pressure just before break thru and use a curved piece of wood as a backer block on the inside. After every hole clean the plastic from the drill bit so that you don't push it into the next hole and there you go. My friend's job was to hold the backer block inside the mold as I drilled. With me drinking wine as I was doing that I surely didn't trust myself to hold a piece of wood under a drill bit, lol.