*thinks* I belive that there are a few patterns out there that cover mascot costumes. (Big headded animals.) Simplicity is coming to mind, might want to check out a pattern book.
I know when I worked as a dancer in a theme park they made theirs out of a combination of chicken wire, some sort of thin plaster and fur over it. But I had to wear one that was 30lbs, so I don't know how much I'd reccomend that one....
Beyond that, try one of these, you can possible adapt one of them. HTH!