Here is an updated plan for the Wolverine Coal Loader insert showing all 9 marbles and 17mm diameter. One of the common sizes for marbles is 17mm and there are many dealers that specialize in marbles of all sorts of sizes and colors. I purchased black ones to simulate coal but other colors or fancy swirled coloring might even look like more fun. Kids love to load the marbles and I am still fascinated by the physics involved in its automatic operation.
Another solution instead of the insert might be to use a 3/4" tube of some sort that would line up the marbles in a row.
