I recently went through this and found that the previous owner used small rectangular pieces of starboard.
On the sides, the pieces run horizontally, and at the fore/aft, they are smaller bits that run up and down.
I cant tell if he planed them down, or just used different thicknesses, but the end result is really nice. Since the starboard bends, the mast is supported at the partners by a wide surface area instead of smaller shims; could prolly do the same with wood, but starboard isn't going to rot.
As it turned out, he got the mast to the prescribed location, and nice and straight, so I didn't have to change anything.
This may be a bit of overkill, but is definitely a slick way of doing it.
-dan
