I tested them in my ID3. More testing will come in the Model y.
Bottom should be also closed.
In almost all race cars they try to cover the tires side with the front bumber. They dont need to think how badly people drive over the curbs and brake the bumber...
Most of these spats are not coming to the outer edge. If its flat plate it might be best choice but more you try to make smooth shapes more towards outer edge you should bring the deflectors. Same problem as with trucks roof spoilers. Easy to direct air too steep on the roof...
I see thing like the tire is a shredder where all air must directed away as much as possible. Think about cars underside and front bumbers.
If you are driving old american car with dirty underside, its better to have a front air dam than smooth round front bumber. Same principle for the tires is what I recommend.
I know some low drag solar racers dont have such a shape, but its still D shape and the tires are much narrower and usually round like bicycle tires. So its not exatly the same situation. Meaning they do direct air to center of the tire.