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Build a mold and a huge oven? Seriously?
Chances are you don't want to make a straight 90 degree duct. Tapering puts you at odds with the "grain".
It can be bent easily without slitting by using a heat gun. Heat guns are also very easy to regulate; by only applying the minimum heat necessary to do your shaping you can keep warpage and rippling way down. Even somewhat compound curves can be formed... but convex only, they get rippley on the concave side.
Chances are good you'll be better off going multi-piece and using something to join them together. I say that because chances are your edges will be curves and not straight lines. Good luck making a one-piece bend with curvature.
Coroplast has butt-ugly edges anyway. This is your chance to cover them up. I've used plastic extrusion for covering edges. In the future I may use it for joining too.
I've used screws with washers, spring-loaded wire retainers, you name it for holding that stuff together.
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