Sam,

The reason I made the 2nd cut is the 1st cut was too close to the edge. There is actually an angled lip molded around the seam of the boat. You can see it from underneath. I stupidly cut into that lip when I should have just measured and cut about an inch further inward. The core edge by this lip is actually angled about 45 degrees, rather than the outer core edge by the lip being vertical.

So - look underneath to see the edge by the seam and measure the width of the lip, then mark topside and cut there. You can easily dig out the core if you have a little overhang of the skin over the lip.