[Phantom]

To fix the sagging and some weight with the what you have I would trim a coat hanger and coat it in glue then push it through the coroplast. You could even bend the coroplast a little against the sag to help hold the shape.

One thing you should check is how the grill works with the fans running at full speed. You do not want to be stuck in 5mph traffic then have the fans turn on pulling the grill shut.

I like the idea and now need to think of some way to do that on the last 2.5ft x1in opening left at the bottom of my bumper.