Skeeter gets the credit for what he did, not me...
My car comes stock with a belly pan / undertray, but mine was uh... deleted by a 16 mile "drive" through a mud road that I took a wrong turn on... my first day I had the car.
I've tried replacing parts of the original with coroplast (plastic cardboard, like what they use for election signs), but back in the summer, a roadkill porcupine or raccoon tore it off. And just this sunday, one of my corners I'd made to fill in ahead of the wheel/fenderwell was taken out by A SMALL BAG OF GARBAGE some moron left out on the highway. *Grumble*
Long story short, coroplast is good for temporary means, but if you have any qualms about how sturdy your belly pan should be: check with skeeter on how he made his tray.
Polycarbonate / Lexan is much more resistant to impacts, and he may have used more than just zip-ties that I used on mine so far.
And I envy that you won't have to lay down in the snow to work on your car like I do, until mid-may!