Here's a tip regarding the coroplast and any hot metal bits like your exhaust:
Use spray contact glue/cement, and hefty aluminum foil.
Shiny side up toward the hot spot.
On a coroplast mockup of my front skid plate below the engine, I had the same concerns with my car, since right behind the oil pan was the exhaust downpipe, and a turbo ...
I foil lined the back half of the coroplast "plate" to shield it from the heat from the oilpan, and from any of the exhaust bits hanging down immediately behind.
The only reason I'm not using it now... a raccoon carcass that I spotted last minute at the crest of a hill on the highway knocked the whole works off.
But I had no issues like melted plastic, nor the car spontaneously combusting after a long run on the highway in summer.
Mind you, I did not have my car completely sealed up from down below, so ... you may still want venting in around the exhaust, yet.
Even the heat shielding my Jetta had above the exhaust wasn't terribly thick ... maybe a bit more sturdy than the tin-foil trays you can get when you don't have a roasting pan for a turkey. So, in my opinion, and in testing nearer hot spots, heavy aluminum foil should prevent firey-commuting-and-hypermiling.