I was thinking of having two (planar) curves, the fat curves, offset from the center curve.
For the tapered section, say the last 25% of the center curve, twist the two curves, using the center curve as a curved twist axis, by 90 degrees, into the
z-axis, and maintain distance from the center curve. The twist would be a tiny incremental rotation for each successive distance along the center curve.
(There is no script for this.)
Project the curves onto the plane.

- Brian....