My boyfriend has got one huge, green fatboy bean bag, which he also kept when we moved in together. I'm not such a fan of this apple-green monster as my cats are. They were crazy about it. After some long discussions my love allowed me cut this thing in pieces and make two much smaller and quite nice cat lounges out of it. (Which fatboy also offers - for 'just' $99!) I think it is a big improvement now and much less prominent in our living room.
Tipp: The best way to empty a bean bag is to put it into a duvet cover, so this tiny little, very light filling bubbles don't make your living room change into a winter wonderland if you just sneeze once. Also a duvet cover doesn't facilitate the static charge of the styrofoam balls as a plastic bag would. And it is big fun for the cats, if you let them anywhere near it.
The cat bed measures 70x40x15 cm. I first sewed a zipper and velcro fastener to one of the small sides.
Then I sewed together the sides, before I connected them to the top and underside.
A better description (in German) how I made it can be found here: http://fraushizzle.blogspot.com/2012/01/fatboy-slim.html