This is the old water tank I built for the roof rack of the 'Cruiser. The part that is visible in the picture is 6" PVC pipe. Connected to that is a length of 2" PVC pipe that sat between the rack and the roof of the vehicle. The 2" section is what held the tank in place. There was a spout on one end, and the top of other end could be opened for filling and allowing air in.

The problem wasn't so much that it was heavy (or that it looked like a rocket-launcher), but rather that I sealed the pipes fairly poorly. The water came out tasting like PVC cement. Didn't last long, but entertaining.