More on my fresh air intake for the HVAC Dakota box on my 49 Dodge.
Hole cover: The piece of metal started out as a 9" x 6" piece of 20 gauge. It was hand formed to the curve of the cowl. After the curve was done, both edges wire trimmed to match the sides of the hole. The top was bent to fit against the top edge. While being held in place, two holes were drilled in the bottom overlapping edge and self taping screws were screwed into those holes. Both sides were primed. The under side had a small piece of the silver covered bubble wrap glued to it. When the truck got painted, the cover was already screwed to the opening and was painted with the top of the cowl. When in place, with the hood open it pretty well blends in.
Pic 1 As would be viewed from the passenger side of the truck. As you can see, great care was taken to assure the utmost beauty (not). This rolled edge fits over top of the round rod that was welded on the cowl as a water barrier. It and the screws locate the cover.
Pic 2 This is what the bottom side of the cover looks like, from the same edge.
Pic 3, Top view of the driver side of the cover. This side buts up against the opening's edge.
Pic 4 is the bottom as viewed from the drivers side.

Winter fresh air, with a summer cover, crude, simple, and effective. Gene