Whenever I need to make an arc in a project, I’ve had good results bending a thin metal ruler to the desired curve between two brads. The trouble with this setup is locating the brads to avoid putting holes in the project or my workbench. The brads can also work loose while I’m drawing the arc. I solved those concerns by designing the marking gauge you see illustrated here.
The gauge consists of a long beam with two sliding stops. It works using a sliding dovetail joint. The “tails” on the stops slide along the slot in the dovetailed beam. Each stop holds a 1/2"-dia. dowel pin to support the metal ruler when it’s flexed. To keep the stops in position, the pins are installed in the upper corners of each stop. As the ruler bends, the ends of the ruler push against the pins, wedging the stops in the groove.
Using the gauge is pretty straightforward. Start by clamping it to your workbench. Then, flex the ruler between the stops, adjusting them until the arc is at the desired curve, and draw the arc. The jig works best if the workpiece is as level with the jig as possible.