I saw one of these in Lee Valleys email flyer and it prompted me to show this one I made 30 some years or so ago. They call theirs a Drawing Bow and I call mine a Radius or Arch Marker.
That old worn out or broken wide band saw blade, you will probably throw away will make a nice tool. Cut off a nice length or several different lengths and grind the teeth down or just leave them and cut a single shallow notch in one end in the middle and another in the other end with an extra one between the tooth edge and the center. Get rid of all the Burrs so the notches are smooth .
You can use string or masons string line a little longer than the piece of bandsaw blade and tie a knot close to both ends.
Reverse the natural bow of the old Band Saw Blade and and slip one end in the single notch, Bow the blade to whatever Radius you want and slip the string in the middle notch of the other end and wrap it several times in the two notches to secure it. Lay it down on your work. Lay a scantling across it so you can hold it down with one hand and them mark around it for your Radius.
You can even make small radius tools from narrow bandsaw blades.