Step 3: Measure the Slots
Here are the critical measurements of the GoPro's mount as I measured them.
Gap Between Legs: 3.5mm (a bit variable at the tips due to to their flex)
Width of Legs: 3mm
Diameter of Hole: 5mm
Slot Positioning Method 1: "High Tech"
Most clamps have a useful line down the centre from the moulding process. In order to place the slots accurately, measure from the centre line to the centre of the slots. This means, the centre of your slots needs to be (Gap Between Legs)/2 + (Width of Legs)/2 = 3.25mm away from the centre line.
Measure 3.25mm from the centre line in a few points with a vernier and make marks in the plastic with the tip of your blade or a pin. You should end up with two parallel rows of marks, 6.5mm apart.
Slot Positioning Method 2: "Low Tech"
Draw all over the edges of the GoPro arm's legs with a felt-tip pen or highlighter, then roll the legs on some paper to get a trace.
Cut this paper out and stick it to the clamp with a bit of clear tape, then poke through the paper along the edges of your traces with a pin or blade to make marks.