I had to cut shims that had a curve on one face, but with the regular flat face on the other side. Cutting regular shims using any kind of fence is complicated enough, but the curve mine required made using a standard fence totally out of the question.
Without a fence, it was trying to hold the 1-1/2″ thick wood at a constant ninety degrees to the table was a futile effort. The result was shims that, more often than not, had inconsistent and unwanted twisted compound bevels.
To solve the problem, I merely held the piece being cut against a two or three inch wide block of wood having about the same dimensions as the piece I was working on. Essentially, the block acted as a free floating fence and produced consistent bevel cuts, regardless of if I was trying to produce a regular shim or one with a curve.