I tried something different for closing the grill openings.
The cure all Duct Tape!
Half width fits between the grill members and black is hardly noticeable.
I have been driving this way for several days and am amazed that the duct tape strips are staying put!
I have seen my mpg go up, but since it has also warmed some here I cannot say exactly how much.
I left the top slot open but covered the rest including the lower all black grill.
Checking the water temperature it still is below 1/2 on the gauge often, so maybe I can do more, but am being cautious.
Good Job but you need to close off the hole thing, you're safe to 70deg.F anyway. If the temp gauge doesn't go to the middle it means you are not getting to180deg.F and you need to get to 206deg.F. If you are worried get a ScanGauge and you will see exactly what temp you are at. You wouldn't believe how cold the ICE runs. Then you will want some Grill Covers.