This tool lets you easily mark the center of an object, like a piece of would. It also allows you to mark lines at specific size insets to the edge of an object. It is based on existing tools you would find in a hardware store. I designed this in Fusion 360 and have uploaded the files to make it easy to change / customize. The model comes in two parts which can be printed individually to avoid supports. You can also print them as a single unit with appropriate supports under measuring surface.

The "towers" on the top have a couple of features in addition to their use in the center-finding process. First, by placing 12mm diameter magnets in them, the tool can be stored by snapping it onto a metal surface like a cabinet. The towers also have holes that allow you to store a pencil (see photo).