[Sven7]

Block off the top grille if you're going to do a partial block. It will facilitate cleaner airflow over the rest of the car. If you're worried about cooling try a cardboard one taped on at first and once you've deemed it safe you can graduate to coroplast. Then start on the outsides of the lower grille and see how much it'll take before running too warm. It'll only take 30 seconds to rip it off if the car doesn't like it!

The belly pan can be coroplast as well. I've zip tied mine to existing holes in the bumpers and subframes and you should be able to do the same. Be sure to leave access holes to change the oil and filter. You can tape a little flap shut to keep it smooth in the mean time.