Around here, they're called "Winter Fronts", and they're a big part of the Truck Customizing business. High profit, low risk, and they can hire a high-school kid to do the actual work of installing the snaps around the grill that the fabric snaps to.anyone ever use a Winter Grille Cover, not sure how much that might lower transmission fluid temperature.
Winter Fronts are substantially LESS efficient than a sheet of cardboard cut-to-fit and placed BEHIND the grille, against the various coolers directly.
In the old days, they'd position the cardboard right on the front of the radiator. In my time, it got put on the A/C condenser. Now, with stand-alone Trans coolers, oil coolers, PS coolers, auxiliary fans...it's getting hard to find enough space for a sheet of cardboard.
Point being, a cover over the grille blocks some wind, and does some good--but there's still plenty of air coming up from below the grille. A sheet of cardboard directly on the heat exchangers is more efficient at blocking airflow. So much so, that you'd need to cut a hole in the cardboard to allow SOME air flow.