I thought about getting the square holes water cut locally but I read on the Vans forum Paul Dye said "If you can build a plane you can cut a square hole" he then proceeded to give some tips which I followed/amended.
After step drilling the corner holes I used a metal blade in a Stanley knife and it worked a treat to get close to the desired line..
I then used a steel ruler hinged with duct tape - I can't take credit for the ruler (Paul Dye and my Inspector) but the idea of the hinge is mine - tape it so you can see about 5mm. When your file reaches the steel ruler it lifts it up so you know you have made it flush. The steel ruler is the key to getting a really nice straight line. As the EFIS will hide this hole this was a good one to practice on but I was really pleased how well it turned out.
