[Pumpkinprincess]

We use cheap holiday inflatables. Buys large pumpkin or a Christmas ornament, etc. Take out the motor and lighting and fill with newspaper, styrofoam, etc. The material is very strong and weatherproof. You can use it again and again, if you want to modify your project. I often find them for free or cheap on Craigslist.

 

[Pumpkinprincess]

Just reread the OP. I see you need it hollow. You could inflate it until your outer shell hardens, then deflate and remove, like a balloon for paper mache.