Here's some pics. Admire them!
All sheet metal, as I actually have an easier time getting that than coroplast. Hoping it doesn't rattle, or vibrate too badly, or make the engine overheat. Monitoring temp is going to be a prob. The car has a gauge, not an "engine is hot" idiot light, but the gauge doesn't work about 99% of the time. Some continuity prob. I'm leaving the lower openings unblocked in hopes that will prevent overheating, and that if it does get too hot, I'll be able to tell from other hints, like, oh, antifreeze shooting out the front or some such.
As to making this a real experiment, that won't be easy. I can't use a Scangauge on this car-- too old. Interpolating from gallons added at a fill up seems too uncertain, with so many variables, but at least easy to do. Perhaps a roll test on some empty bit of road (might have a chance at 3 AM), where I accelerate to a certain speed (70 MPH I suppose) and then at some marked spot start a "cold coast" (engine off coast, tranny in neutral) and see how far the car rolls with and without the grill block? Or I can do it the lazy way: forget trying to measure the effects and just take it on faith that it works.