I have been successful using vertical nipples and my horizontal nipples. I used a 21/64 drill bit and then used a 3/8 x24 tap to put threads in the material. I've had no leaks and have used PVC as well as plastic containers. If you are cracking the material, you could try drilling slower and predrilling with a smaller bit, then step up to the 21/64 drill bit before tapping the thread.