Learned that lesson the hard way too, except mine was a back plate that split down the middle after two wears. Now I do a 6mm foam base, hit it with a heat gun to shape it, then skin it with two thin layers of Worbla instead of one thick one, and I leave the inside of the curve bare foam so it can move. The key is not gluing the Worbla to the foam across the whole surface, just at the edges, so the two materials can slide a little when you bend. Mine has made it through two summers of con floors with zero cracks since I made that switch.