OSTexo just want to thank you for 2-Rail Sweep suggestion :-) I ended up using that method and it really was the easiest, plus it gave a surprising amount of control of the shape through-the-model via the profiles.

At the end of my build I tried to round-out (fillet) the interface between fin and fus, but its difficult geometry and in the end I was happy just to leave it. Below are images of the end product, its being printed in PLA and ABS plastics as I type. Printing required splitting it up so my print guy could get it on his print bed, plus hollowing it out saved some cash.

Cheers and thanks everyone for their advice and tips. Michael I'm singing MoI's praises to my fellow glider builders, you may see some more interest from this hobby segment :-)

BTW chrisd - good thinking on crosswinds affecting the big fin. I had to think about that one as the answer is no, but why? The reason I believe is that the glider flies WITH the airflow (wind), not in competition to it. This means there is no "crosswind" to a glider. The tail is there to fight the glider's inherent aerodynamic imperfections which are magnified by a wing-only ("plank") glider. As the wing moves through the air, imbalances cause see-sawing which the fin dampens out. Hope this off-the-top-of-my-head explanation makes sense.