Have just done it frehand with a dremel on my IB-5 the other day... It can be done - take your time and don't rush. Used a router bit - 650 - straight cut, then clean up any edges with sandpaper and a small chisel. Big issue I found was that with the supplied cover, most of the screw holes sat over voids. In the end I had to oversize the area that I cut out. Second challenge is that it's not flat and the edge dies away to the edge of the guitar. It can be done - you may be more successful with a template, but it may take a bit of doing to get one that fits on the edge.
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