Cellfoam 88 comes in multiple thicknesses. I found 3, 5, and 10 mm sheets at the Hobby Lobby. Not only that, but at least in Kansas, the store is having a sale. I got some wood for robot bodies at 50% off.
There's a tape for gluing Cellfaom 88 together. Don't use that for a BalloonSat airframe, instead use a bead of hot glue. Then cut out any ports and openings. Afterwards you can wrap the airframe in thin colored tape.