I enlarged the hole on the left side primarily with a router, first with a router bit having a ball bearing on the bottom which cut a notch in the side, then with a bit having a ball bearing on the top (bottoming bit). That way, I always supported the router with a ball bearing resting on the edge of the workpiece.
You might notice a slight gouge in the upper right corner - that was due to the earthquake we had yesterday while I was using the router (at least that is the excuse I am giving).
I squared off the holes with a multi tool.
For the right-side, I used the multi-tool almost exclusively as the ladder was in the way and would not let me use the router much.
Basically, I had to enlarge the hole by cutting out about 1" on the top, and squaring off the corners.
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