I scratched my head for 10 seconds, and then I remembered where these old Stanley trammel points were:
I found them in the box I bought a couple of years ago, of course!
The 8-inch beam was not going to help. So I decided to make a new one, long enough to trace a 27-inch radius. A sharp block plane does a nice job of rounding the sharp edges:
A few minutes later I was using the trammel points on their new beam, to trace the arc on the boards destined to become rockers for the cradle:
A little shaping at the band saw and at the oscillating spindle sander, and the rockers were ready for a dry fit. First, the shot from the foot board — the reverse curve on the ends helps to prevent rocking too far and tipping the contents:
And also a corner shot, to provide a view of the matching rocker at the head end:
I cannot think of a better way to trace long arcs like the ones required for the rockers. Any length beam will do; I like them!