Yes cutting the window was a challenge. I drilled it close using a small drill then joined the holes using a dremel router bit. The same bit was used to hollow out each end of the slid an a bit to the sides to allow the button to slide. I have made the scales slightly thicker ( see photo) to hopefully give them enough strength.
The whole thing took the best part of ten hours. My top tip is to scan/photocopy your scales then glue this to some ply. This gives you a template which is flat both sides when it comes to the drilling and cutting.