Coroplast can be bent with the grain or up to about 30 deg either way by slitting the inner surface. Use a blunt-tip knife to save the other surfaces, and hold it at an angle so the cut edges slide over each other. For other forming, it is best to heat just the inside with a hair dryer or hot air gun. Sharp creases can be made by scoring with a blunt tool, and then running a bead of hot glue down the groove. That will first heat the polypropylene, and then help hold the shape. Oven heating can give full vacuum forming options, but you will probably make a large pile of scrap getting the process adjusted.
BTW, I have actually done lots of this stuff, not just guessed at it. Silicon seal works for bonding - I've never seen "plastic epoxy." Polypropylene is among the "waxy plastics" - hard to glue.
COROPLAST HPV BODY CONSTRUCTION