These devices make it easy to draw circles on a standard battle mat.  Three versions are provided: a dual 20/25mm scale "Y" shaped unit compatible with both older and newer Chessex battlemats (making circles or arcs with a radius of up to 4 squares), a single 25mm scale version with the same maximum radius, and a longer version at 25mm scale that has a full 10 square radius which includes a straightedge for line drawing.

These tools are designed for use with wet or dry erase markers (as required for your specific battle mat).  I printed the smaller ones in PLA with an 0.4mm nozzle at 0.2mm layer height and 15% infill, and the longer ruler unit using an 0.6mm nozzle at 0.15mm layer height.  The pivot “bearing” has a slot in the bottom which can be used to help break it loose; the longer version has looser tolerances needed with my 0.6mm nozzle, so I can't recommend printing the smaller ones with that size of nozzle.