For aero panels below the car you might still want to use Coroplast because its so much lighter than other stuff and holds up very el. Mine has about 50k miles on it now. I do recommend 1/16" or sometimes 1/8", by 1" aluminum bar stock for framing for Coroplast. It's too floppy all by itself for big panels, my opinion.
To fasten panels to framing I used 1/8" diam pop rivets with washers under the flared heads. If joining panels together use 1/16" or more bar stock on back side for the rivet to grab.
That said, the FRP sheets might work well. Just make sure they are not adding unneeded weight.
Coast long and prosper.
Driving '00 Honda Insight, acquired Feb 2016. On hold: plans to purchase an Elio enclosed reverse trike, due out some time in 2018.
We've learned that diet has a closer relationship to than we thought. Please see this Wikipedia article on The China Study