[RedDevil]

First results, if it can be named so, from the wing shaped antenna sleeve.

Arriving at work after 35 km the rear edge had split open in the middle; apparently the glue failed. The sleeve had slid up slightly and the point of the antenna stuck out of the gap. So I took it off.

Version 2 had internal bracing to give it a better wing shape, all in bagel box plastic, but it meant more glue was needed to keep all in place. Tried to speed up the drying time in an oven, but overcured it so it deformed enough to just bin it.

Version 3 had internal bracing by simply adding extra folds to create a triangular shape inside the wing, providing a large glueing surface while mimicking the ideal wing shape quite well.
I blew against its leading edge with the antenna inside, and against the antenna without the wing, and sure enough the wing did reduce drag noticeably even in this crude handheld windtunnel substitute.
Looked much better on the car too. Stuck quite firmly to the antenna.
5 km on the highway, doodling along between the semis, I heard a buzzing sound, then a nasty pop (even felt that, but that might be imagination) and off went the plastic sleeve, onto the A12.

So even though the stalk is quite short on the Insight, the force on it is substantial so any aero work on that does bring benefit and needs sturdiness.
Version 4 will be secured firmly, that's for sure.