why not just use large sheets of foam and carve it out? I could see chicken wire for corners and arches but instead of foil I'd use chicken wire, and spray foam (great stuff makes a fire retardant foam). I'd get the thicker pieces and use them most from home depot are 4x8 and come in a varity of thickness's. then you cover it in monster mud and texture it however you please. the end result should be a fairly light weight structure that would be safe for people to touch as long as they aren't pulling on it. Plus it can last for years. Now if this is going to be just for a single years use then I'd say yes foam is your best bet. But if you want something thats going to be up and down year after year spend the extra time and money for a better result. Just my opinion.