It's all a matter of accurate layout and marking (if you're not so accurate). I have a CI-Mp wheel with 10" pin holes. I've made plaster bats, corian bats and bats from old Formica topped particle board kitchen counters. In all cases I have marked the bats on one edge to align with a mark I scribed on the wheel head. This way I can reposition the bat the same way it was on the wheelhead when the piece was thrown.
In making the corian and formica bats, I rough cut the material, layed out the center point and dinged it with a SHARP center punch, and layed out an accurate 5" radius circle. I drew a line through the center point to intersect the circle on 2 sides, carefully punched the bat on the intersecting lines and drilled the bats first with a 1/8" bit and then with a 3/8" bit to match the bat pin heads, also being careful not to drill all the way through the bat. Test the piece BEFORE you cut the final circle so you don't waste a lot of time if you screw-up the hole drilling. You can layout the final size to whatever you want the bat diameter to be.
The particleboard bats were painted with 3 coats of spar urethane to waterproof them so they didn't swell when they got wet. No urethane on the Formica side. You can generally get the Formica and Corian sink cutouts for free from a place that installs kitchen counters, or you can contact a friendly kitchen remodeling contractor and ask for the pieces. You can usually get at least 2 bats 12"-14" in diameter from each 21"x30" cutout.