I finally decided to have the Harpoon separate at the nose cone. To keep the pressurized volume down, I built a stuffer with some suitable found tubing and a thick card stock ring. I had already cut the ring but had to do some trimming to make it fit in the coupler. A Kevlar leader is epoxied to the inside of the coupler and is further reinforced with a small square of fiberglass. The leader extends to a inch or two from the top of the upper BT-101. I'm going to use a wide piece of elastic for the rest of the cord.
The slots for the lower fins have been cut and the fin tabs adjusted. Lamination of these fins is in progress. The tail cone has become more complicated than expected and it will take a tad more work once those lower fins are on.
I decided that am going to add some raised trim. Probably triangles of balsa on the roots of the lower fins. Maybe card stock? Some Harpoon variants also have raised attachment hardware on the forward fins. These will be also be balsa...or card stock. Finally, there is some hardware where the Harpoon attached to its launcher. I'm going to try to make this our of 4: mailing tube. If this works out, it will also provide a good mounting surface for rail buttons.
That's it for now!