[Christ]

Got an oven that your complex shape will fit in?

Bend up a section of chicken wire, to use for a form (mold) then lay in a sheet of coroplast. Put it in the oven (find out what your Coroplast is made of, then find the melting temp for it) and heat it up. Let it cool WITHOUT opening the oven, and once it's back to room temperature, it will have a memory of the shape that you melted it into.

To join sheets to make them bigger, there are two ways. To join lengthwise (holes facing each other) you use plastic sticks in every couple holes and then slather some plastic epoxy between them, scrape off the excess when they're mated.

For the other direction, you take a couple old wire coat hangers, straighten them out, and bend them entirely in half, with no loop at the end. (So that the length of the hangers is parallel as good as you can get it.) Cut a notch into the corrugation of about 1/2" on either side of both sheets, slather the hanger with plastic epoxy and slide it in. Brush epoxy down the length of both sides of the seam and let it harden up.

(Plastic/Rubber cement also works.)