I made my own grill covers out of clear Lexan from the local hardware store. Cut easy with a coping saw or a table saw for long cuts. Being clear, you can't even tell they are there.
After doing some searching and reading I learned you weren't supposed to cover more that 80% of your grill openings (including the bumper). So, I made my grill covers with a 3/8" gap all the way around. This provides the needed flow, but it is well distributed so you don't get cold spots for thermal fatigue. On a 2nd gen, I just used 5/16" carriage bolts with wing nuts on the back, easy on, easy off. The bumper slots were more difficult and I only got them made last weekend. So, I haven't gotten any good testing on the whole system. I did notice that just having the upper 4 didn't seem to make a difference. But looking at the size of the bumper cutouts, they flow a lot of air. That maybe why it takes the 12V longer to warm-up.
I drive 9 miles to work, so the engine is just reaching normal operating temperature when I pull in.
I made a set for my brother's '93 and he definitely noticed the difference in warm-up and operating temperature.
Just my experience so far.
- Dave