This is a spherical die, that uses a weight inside to pick a number.

At 30mm size, you should use a 3/8 inch metal ball.
At 25 mm, use 5/16 inch metal ball
At 18 to 23 mm, you can use a BB or slightly larger ball. (note, I have not tried it this small. 25 mm is the smallest I made.)

When the print is about 50% - 60% complete, drop the metal ball in, and then finish the print.