Lund used to make winter cold fronts for a lot of vehicles. It was basically a thick nylon or plastic flat sheet about 3/16" thick that had holes all over it. You could do this and put it behind your grill and side vents. You might be able to add some longer bolts that hold your grill on and then just slip these covers over those bolts , then attach your nuts.

I deal with his problem with diesels every winter. The diesels have such large radiators and the cooling systems move so much coolant that it is hard to warm them up and maintain heat in the cooling system when it gets cold. Granted, it usually isn't an issue until you get to 30* and colder but it is an issue.

The cardboard trick is a good way to do it. The cardboard may be ugly but if the temps rise outside, it isn't very hard to take the cardboard out and set it behind the seat so you don't over heat the engine.
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