Hello & welcome to the forum,
I don't know anything about building a float tube out of styrofoam........ There are many affordable tubes on the market today; why are you looking to make one? I don't own a tube and only fished from one for the first time this spring at a Casting For Recovery event. What little I learned is that you will want a decent one if you are going to be in deep water or be staying in it for a few hours at a time. Take a look at the Sierra Trading Post to see what they have on sale.
I just took a quick look, they run $109 - 169 http://www.sierratradingpost.com/fis...essories~d~56/