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.
Dave Shaw
EAA 67180 Lifetime
Learn to Build, Build to Fly, Fly for Fun