Science project, I want to make a whale to-scale, what would be the best kind of material?

OK so for a science project, what is the best stuff to make a whale out of? I want it to be sturdy -- not hollow. OK, so like, do you think styro-foam? , or paper-mache? or molded plastic? but how do i mold plastic? which would be the cheapest material because I can only spend total of 20 dollars. The whale can be as big as I want.