I was recently asked about the above decoys that I made approx 18 months ago, so I'm putting a photo back up   Correx is the same type of material that's used to make 'For Sale' signs etc. I was lucky because a family member works for a company who uses this material, and he gave me a load of offcuts.   Wickes sell a product called Proplex, which is a thinner version of Correx, and can be cut to shape. I recommend two pieces glued together for added strength.   Here