Good luck David
To serve science I suggest you don't add an endplate to the rudder at once, but if you make the low end squared off and horizontal, you can glue on a plate later. Then you can better learn what factor improved the steering most. My limited experience is that on a low aspect ratio rudder, an endplate will improve the rudder more than any careful streamlining of the rudder itself will do.
The late American boat designer, Phil Bolger used the endplate on many of his designs after having tried it on a traditional catboat. I have used it on a sailing row-boat and it really works.
Arne
PS: All my jabbering about rudders here may sound a bit off-topic, but it is not: Better steering lets one get the best out of the sail with less fuss.