These things (from various manufacturers) are available at prices as high as $140. So, here's the other end of that cost spectrum.
parts needed:
(2) - general purpose clips (also suitable for sealing a potato chip bag)
(2) - nylon screw (1/4"), see photograph
(2) - nylon nut
(1) - bit of plastic sheet--the shape and size is up to you. The lid to a plastic storage box works well.
I cut the plastic using sheet-metal snips and made the hole using a paper hole-punch (1/4"). Really.
I used nylon hardware as I didn't want any metal to possibly mess with the Stelvio windshield.
Note the additional round grip near the hinge of the clamps (see photograph). This fits the thickness of the windshield, perfectly. It attaches easily, and is quite snug. It isn't going anywhere. The angle of he clip deflects the air higher from the end of the shield.
assembly:
and, mounted:
I suppose black would have been less conspicuous for the clips, but really, it isn't too bad--certainly, no worse than the $140 version. The clips could be sprayed flat black in any case.