Bill_G_62439
Observer
Stereo, with the tools I have, I would mark the complete square where I wanted the hole on the bracket. The I would start with a small drill bit at the center of the square to make a pilot hole. Then I would use the biggest bit possible to stay within the confines of the hole. Then I would start working on the corners with a dremel tool and appropriate bit. Alternatively and finally, I'd finish with a small file to define the corner.