[Bicycle Bob]

Sorry I'm late to the party here, but I've used a lot of Coroplast. Big flat panels are problematic, but if you can use a conic section, with maybe 30 degrees of arc across the width, it stiffens right up. For a flat section, you might make a series of bends to give a shiplap effect. You can also stiffen it with more layers, or by ordering the thick stuff. Non-paintable silicon seal is a good adhesive. I have also bent it into hat sections to add ribs, and used strips of unidirectional fiberglass for hard points. For a monocoque section, just adding extra layers near the mounting points helps a lot.
I'd always start by trying for a monocoque, and reinforcing that, rather than building a frame and wasting most of both it and the covering. The peak loads are gusts, and a bit of calculated flex can handle that with far less material. However, if you don't melt the ends of the flutes to seal them, you will hear people calling it cardboard.