I think those folding tabs are a good idea. If you could install 90deg angle iron brackets along the folds, it should make it pretty sturdy.
I've made some RC airplanes out of the stuff.
Coroplas is like corrugated cardboard and is pretty easy to fold along the lines of the corrugations and folds with great difficulty sideways to the grain of the corrugation tubes.
A lot of people find it so hard to fold, even just to 90 deg, they cut along one side of a corrugation tube. I don't recommend that.
Lay the coro out flat on a hard flood and take a dull screwdriver and use it to
compress a line down one of the corrugated tubes. That will allow you to make a clean fold. Sometimes it takes a few passes with the tool to bend the plastic into the tube enough to get a clean fold.
If the wind load is high, the coro will bend. So, for wings, we install steel rods inside the some of the tubes. (Music wire from a hobby shop).
If you make the L brackets (or angle iron brackets) large enough, they might do the job of keeping the coro from flexing under with the wind.
You could do a small test install and have someone drive in front of you at 65mph take a look (or video) under your bumper.