I don't think that I'm giving away any secret by saying that the way to achieve the proper shim shape is to have the same radius on the inside and outside. That radius should be whatever the radius of the rocker shaft is. The tapered ends happen automatically. Stack two washers of same diameter on top of each other and offset them .020" and it becomes obvious. Creating that profile in the shim would require a milling machine or lathe and a bit of machining knowledge. Shims made this way would theoretically seal as well as if they aren't there because all the radii match.
