We get this stuff thru work (aircraft manufacturing)...I don't think it's too pricey...?
I think aerodynamically, blocking the top grill will smooth things out a bit...?
I've noticed the heat coming on sooner today with the block, but it's relatively mild today (2 Celsius ).
I wouldn't want to close off any more than I have now.
Big road trip coming up over the holidays...I'll have to monitor temps and mileage