This is my first go at shaping a tri plane hull and I am very please with the outcome
It is quite fine and has quite pinched rails all round
The Greenough 4a fin should do the trick
It has only a very slight rocker and a rolled bottom that blends into a more pronounced V through the tail and the fin. It also has a flat planing surface most of the way that create the tri plane contours and it should by rights sit and plane on that surface.