The arc marker is an ideal tool to mark the 180° front arc (90° marking is also possible) on bases.
Simply push the base into the slot (25mm, 40mm or 55mm), hold on to the base and the marker, and draw a line along the clip. These clips have the same angle as the edge of the base which makes marking the front arc easier and precise. You can make the marks using permanent marker, a brush or a pencil (which is removable).
You can glue the clips into the marker or you just plug them together. So you can remove the clips from one side in case you are using a base that overlaps.
If you want to glue them together, it is recommended to use wood glue, so that in case you're using acrylic glass you have still the option of removing the clips.
Due to the tolerances of the thickness of the material, it may be that the clips need to minimally reworked or glued. Please do not use force!