Then when you have it shaped the way you like, apply a Worbla outer-layer to get it stiff and sturdy
https://www.amazon.com/Worblas-Finest-Art-Original-Thermoplastic/dp/B07CKB7HP8/ref=sr\_1\_7?dchild=1&gclid=Cj0KCQjwh\_eFBhDZARIsALHjIKcx\_gY1eAbHTVZnZjOIuGievjt-F7k3U7gbUiDOT7DNWCfkIfk2dW0aAl-DEALw\_wcB&hvadid=177539057961&hvdev=c&...
Here's a link