Regular Silicon seal (not paintable) is probably the best, if you can afford the drying time. WEST epoxy is pretty good, especially if your Coroplast is fresh with lots of "static cling" left from the factory bond-enhancing treatment. Premium latex-based contact cement is very good, but may not endure freezing. A quick way to get a neat, even curved crease across the grain is to run a bead of hot glue to soften the polypropylene, and then make your fold, but hot glue is not much good on a hot, sunny day. Always be aware of sharp changes in flexibility, which is where cracks start, either in adhesives or materials.
See
COROPLAST HPV BODY CONSTRUCTION