I think the design of this clamp, as it pertains to the rubber piece and the 120393 PW (washer) is to allow for a prescribed amount of squish (or squarsh, depending on your geographic origin) of the rubber to securely hold the column in place. If your aftermarket rubber pieces do not allow the two clamp halves to touch the 120393 PW when bolts are tightened, I see no reason why adding enough washer thickness between the clamp halves until the halves touch the washers wouldn't be an acceptable solution for you.
