During two weeks I also tested a wiper cover. As it was only a coroplast sheet taped with transparent tape, I had always an eye on it when I was driving with it. While driving it was always very stable, except when a truck was overtaking me then the left part was shacking a bit. I was also inspecting how well it was taped before driving.
I had a "no rain" position (1st and 2nd picture) and a "may rain" position (3rd picture). When driving the coroplast was pushed down and in the "may rain" position it was only at 0.5 cm over the wipers. The vertical sheet in the "may rain" position is simply stuck between the fixed and mobile parts of the hood. It can be put back and removed without untapping the cover.
Not only it removed the wiper from the air flow, but it also smoothed the angle between the hood and the windshield.
My best daily commute consumption since my first message in this thread is 57.3mpg. It has been done with the wiper cover in "no rain" position. The following best daily commute in that tank was 51.4mpg. But this record may have been helped by a very warm 18°C (64°F) day...
With cover 3/4 of daily commutes were over 47mpg while without cover 1/2 were under 47mpg before (with same temperature range and "full" grill block).
In my daily commute, when I leave the freeway I have to pass over it thanks to a bridge. At the top of the bridge I'm at 60km/h. I coast in neutral to go down. Without the wiper cover I reach 76-77km/h max at the bottom. With the wiper cover I reach 79-80km/h. I tried to follow the same curve, parallel to the right line. I did it 10 times with the cover and 5 without.
A funny thing : when it was raining then the drops fallen on the windshield under the level of the cover were pushed down to the wipers, effect helped by the fact I used an anti-rain product on the windshield.
I think this cover is a success
If I put the cover back then I will cut the lower corners so they are not pushed up by the upper sides of the hood. May be that will permit the hood to be opened without removing the cover. I may also cut the cover so it stop just before the wipers, which may help finding a "any rain condition" configuration.
Denis.