I held everything in place and traced the keyhole with a mechanical pencil.
I clamped it to my toggle clamp board and positioned a lamp behind so I could better see the original pencil line. If you want to use a fine Sharpie to enhance the line go right ahead.
With a series of files and needle files I went very carefully around the line to perfect the shape. I found it works best if you angle the file slightly in at the bottom. This will create a slight taper and you can start fitting the bolster piece on and test the fit.
This is the left side scale done. Repeat the process for the right side scale and try to match the left.