I reccomend buying a couple sheets of sintra (expanded PVC foam.) It's pretty cheap, becomes malleable with little heat, and is easy to cut, sand, and paint. Totally a fantastic material for armor making. If there's a sign making shop near you I'd go check as well, some will have scrap material they'll give to you for free. It's harder to make a helmet, but it's doable. Making Boba-style armor out of sintra is a absolute breeze once you get some practice in!