I cut out 1/8" masonite (hardboard) a bit smaller than each size.
I added a few improvements to the process, essentially following the instructions of John Carlson.
I ended up putting on two coats that had the marble powder, but the 2nd one had a lot more powder.
So we'll see how they work. I've made about 450 of these and have changed the process a number of times. I guess I just can't stop experimenting.