The other day, I went to a fabrics and craft supply store to get high density foam and found a 17 x 14 x 2 inch piece. Usually, I scrounge most of the foam I use, but this time, I didn't have any squirreled away.
I had measured my wheel and pan prior to shopping for the foam, so I knew the maximum outside diameter measurement to shop for.
I marked out the circumference of the batt onto the foam using a marking pen, then cut it into a circle using large sewing scissors. I don't really care if the foam is neatly cut--that doesn't matter, just as long as it clears the catch-pan of the wheel.
I put a batt on the foam to serve as a base, centered it, then glued it on. This is done with the foam side down and a hefty weight put on the batt. As my Dad used to say, "Glueing without pressure is a waste of time."
After I let the glue set for 24 hours, I used a long bamboo skewer threaded through the batt pin holes and pierced through to the top on the reverse side.
I marked with a permanent pen where the skewer came out on the top. This way, with the foam side up, I can easily locate where the batt pin holes and center the trim wheel easily.
I marked around the skewer with a permanent marker
I put the new batt on the wheel and marked true center. I measured out from the center in one-inch increments and, holding the marker on each dot, rotated the wheel to create concentric circles.
I like the foam because it grips the clay. it's easy to center the piece using the guidelines, the foam doesn't alter the lips or rims of a form. You only need to steady the piece lightly with your fingers to keep it in place while trimming.
The foam catches the trims for easy brush off.