For that job I would use a router. With a router all your holes will be exact copies of your master template, insuring a perfect fit.
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Never did find out how big this square hole was...
It would make a difference about how it was approached ...
This was drilled at the corners so I could FIND the hole after the Mahogany skin was epoxied in place
The router with a bearing pilot for a guide , made EASY work of cleaning the edges
Later I cleaned the corners with a file.
Gotta Fly...
Try 3/4”x 4 1/4” for starters. All corners are square.
Don't have to cut square holes for stock Fly Baby ribs. They're formed of three separate pieces, joined by a cap strip.
http://www.bowersflybaby.com/tech/template_header.jpg
Ron Wanttaja