Material
Marble or small ball
A rolling piece for a game you design.
Get to know it
Choose a smooth ball and make the game around its actual size. Test the surface, corners, wall height, and route with the ball in place before attaching everything. A bead with a hole may roll differently from a solid marble.
A larger suitable ball may make a different game more accessible, but the whole board and walls need to be redesigned around it. Keep loose balls contained in the tray.