[mistercrash]

It's just a scrap piece of Plexi. I just trimmed it down to approximately the shape I needed and held it over the rim's cavity to trace the shape. Then I just sanded the edges bit by bit until the piece fit in there loosely. It has to drop in there without being to loose. Finally, a hole in the center. I just eyeballed it because if you put the piece the same way in each cavity then the holes will be evenly spaced all around.