Rudder for 24' runabout and model

Hi. I`m building a 1/7 scale model of my future coldbaked runabout. I`m in search for drawings/suggestions for rudder for fullscale as well for my model, (so it functions for some R/C tests). The model is approx. 1 meter+ long, it has a progressive v that ends up at 23 deg at the rear. The full-scale will hopefully get twin props and rudders, either with two smaller engines or a V8, with the geared-up boxes, that sounds sweet (anyone tried them?).
But the model will have to do with one rudder. Anyone got opinions on sizes on rudders from models to fullsize (I know this is a problem on airplane scale-models)?
Anyone got opinions whether twin rudders always should be at 90 deg to the horisontal plane, or inclination has pros & cons?
I am a thoroughbred rookie in boatdesign, (with a love for impossible missions and curves yet to be put on paper), so please feel free to come with hints and links. Best regards Simone