I have heard of, but have no experience of this one: http://www.windvaneselfsteering.co.uk
It is called the Hebridean and there have been threads about it in the past. A search should get it for you, especially through google rather then the site search engine.
This is a Walt Murray design and fairly easy but time consuming to build. This is mine from a few years ago built from SS and plastic tubing. https://www.youtube.com/watch?v=U66-R0i2c20
This is my current one that is very similar to a Monitor, built in SS mostly. Uses bevel gears which is a more effective method of converting drive than the Walt Murray Z drive. The design of the current one is basically taken straight from an article in PBO a few years ago. https://www.youtube.com/watch?v=isw529SdN8s
There's a book by Bill Belcher called Self-Steering for Yachts or something similar that has good info including plans. It's out of print, but comes up on e-bay.
An in- depth Google search will find a few web sites devoted to the subject.
I used the Belcher book to design and build a servo pendulum one from steel which I then galvanized.
It worked fine when I got it dialed in but the process took lots of time and fiddling with sails to get a balance it could manage. I did not have a remote adjustment on the vane so having a center cockpit ketch made it all a bit of a faff. It was just so much easier to click my old Neco on and rely on the Acquair to keep up with the battery drain.