Scott,
Boats with a spade rudder and no skeg are inherently course unstable. Your boat is, and so is Weaverbird.
A self steering gear for such boats must add the stability that is missing.
The OGT does not do this. A trim tab does not do this.
Both might give adequate steering with the wind forward of the beam, because then the combination of sail, hull and and rudder tends to have stable characteristics (where the hull and rudder alone do not). Neither would work off the wind sufficiently well off the wind to be worth the effort of making them.
An auxiliary rudder gear (the Hydrovane is a commercial example) adds stability to a certain degree, because the main rudder is fixed and acts as a skeg.
Only a servo pendulum gear does a fully reliable job of adding the yaw damping feature that such boats need to make them course stable. And even then, I have found that unless the servo pendulum has an inclined axis so that it tends to trail and self-centre, small boats with dinghy-type spade rudders will not hold a steady course on a run. I have known this since the early 1970s, when I was working with Blondie Hasler, trying to get a Hasler SP servo pendulum gear (with vertical axis vane and horizontal axis pendulum) to work on Pilmer, his Kingfisher 20 with a balanced spade rudder.
That is why I developed the two pendulum gears for which there are drawings in https://junkrigassociation.org/members_files > drawings > David's vane gear drawings.
I have to recommend that either you build one of these, or you take the easier option of installing an electric autopilot. For short passages in good weather, the latter should serve reasonably well.