I made something like these things, of different thicknesses of sheet steel, to experiment with extending the length of throw on a coupla cocks. Didn't taper them though; although a tapered shim might work as a means to diagnose some issues with the geometry of the cock to frizzen, on certain locks.
The "shims" seemed to work ok, but really didn't seem to improve the sparks on any of the locks I used them on.
God bless