Looks good so far. The only comment that I can add is that, if you incorporated a steady curve into the boattail, you could make it shorter and still have the air fully attached along its length. One thing also to consider on the longer boattails is how far they will swing out in the opposite direction when making a sharp turn.
Below is the tail of a Questair Venture kitplane which I used as a model for the shortest possible, fully effective boattail shape for my car's boattail.
