http://perso.orange.fr/ballonsolaire/en-index.htm My dad and I made one out of the thin, home brand bin bags and pacaging tape, but instead of making it look like a balloon, we made a giant, sealed pillow case. We launched it and followed it for 20 kms until it caughtt a downdraft and crashed on a farm. The trick is to make them BIG (about 3m x 2m x 3m)
Fill them with a hair dryer so your not waiting too long until it heats up. These things will float out of sight, following the breeze. One of the ones we made wasnt very good, and it drifted low over the main street and a bikie pulled up on the side of the road to watch
. The most important thing to remember is to make them as close to a sphere as possible, as it has more volume for the same amount of material.