Sacrifice one of each of the surfaces and sound boards that you use.
Lightly Goop two sound boards together for the distance between the top of the pedistal and the shelf for the striking surface. Glass works good so you can clearly see what's going on underneath. Again, just lightly Goop right in the middle. If that is too tight in to your striking surface for you, use one regular weight glass, and one double weight glass.
Take all of these sacrifice peices (they can be ones that were chipped for what ever reason. 70-80 percent round will still even work. Anything over 50 percent in tact really.
Then, here is the trick. Take a chunk of full size roll duct tape. A 1/2" chunk, full width. Imagine it in fourths. Stick the first fourth to the center of the surface. Then fold the two center fourths together. Then attach the fourth fourth center to the surface right beside the first fourth.
This makes a set of true to size, true to material gauges for on the lathe checking, off the lathe checking, etc. and no fumbling of insert or removal to size-check.