Better Customizationable QFN Holders

The script had a lot of magic numbers not tied into the measured values. As such, it didn't work when I tried to update it for the larger PVC sizes I had on hand. Also I added a countersink to the cross config so that the screw could sit flush.

The demo files are for 1/2" (OD 0.840in) and 1" (OD 1.315in) PVC