Step 5: Fit the Lens Mount
Using a pair of dividers or a compass, trace a line onto the PVC, while holding the dull point against the camera face, riding over every bump. If you do this carefully, you will get a line that closely matches the camera face scribed into the PVC surface. An added precaution might be to cover your camera face with masking tape, so that you won't scratch it.
I traced over the line with Chartpak 1/8 inch crepe tape. This helps me see the line while cutting. You could also trace it with a permanent marker.