The following louvre panels are placed on bonnets such as Toyota to allow cooling of engine.

After suitable holes in bonnets are cut out - 250mm-300mm wide  x 100 high approximate size rectangular holes. These inserts panels which are larger than the hole or 350mm wide x 150mm overal panel size - are then to be placed from underneath to fill the holes, with louver openings facing towards windshield. The panels are to be riveted in place using standard 1/8th inch rivets (3.175mm)- so you drill a 3.3mm diameter rivet hole.

The sizes of these panels as mentioned is 350 x 150. The size of louvers themselves are 15mm wide x 250mm long.

Material 1.5mm aluminium.