So, would this be a bad idea for side-mirror drag reduction?
Viewed from the front, the stalk / rectangular catchment area has about 1/2 the mē area of the mirror. In plan view, the 'triangular exhaust' area has just more than 1/2 of that again in mē. It's close to Template C in plan.
Those with access to 3D printers could easily make this. It might need some beefing up in the area where the two connect etc.