Thanks for the suggestion Mike,
Here's what the boattail might look like based on the airplane profile you posted:
My first drawing was fairly close, but the top was too steep at the back. I also just truncated it about where it would have to for taillights, etc.
I just had a thought: the video cameras I am using for side mirrors could be used to video a tuft test of the underside of the boattail! They cost ~$22 and they could be recorded on a laptop with a basic analog video input plug.