First of all, I made a template using the existing mounting holes to draw the cutting line, the shell being cut a little wider that its final depth. I assumed the mounting holes were aligned, which is about right.
I cut the shell down using a pull saw (aka Japanese saw), that has very fine teeth and that allows for a very accurate and fine cut.
Utility knife line on the left for final depth, saw cut on the right |
There it is, piece of cake.