I have the Lund covers, all six.
I cut one of the upper ones in two, one maybe two times larger than the other.
Just returned yesterday from a trip to Coeur d'Alene,ID. Everett,WA. and back.
Going North ( grill never in the sun) I kept ALL of the inserts in place and the gauge went over 190* only at the top of long grades and the t-stat would only do that bounce routine after sitting at idle, the rest of the time resting steady at 175-180*.
Coming South, ran with only the smaller of the two-piece out until Kettleman City where I took out the other half panel.
This is at an average of 70 mph with our 4500 lb, 5th wheel.
Putting them in and out is a PITA, but I prefer looking at a steady temp gauge.
Given my druthers I'd rather have louvers like the 18 wheelers of old that I could manually operate.
And on that note I saw a 1959 KW conv. /sleeper up by Sac. /Stockton that was pretty sharp meester! Still on the job!