CD hovercraft bottlecap replacement

With a CD or DVD, a glue gun or thick glue, a sport drink bottle cap and a balloon you can create a CD hovercraft that will float on a table or other smooth surface. The air cushion is not lifting it very high, but it works quite well.

Due to the high bottle deposit fee I decided to print some for my students instead.
Warning: These are rather thin and for smaller balloons or the long rocket balloons. Increase the larger one by twenty percent to avoid trouble with the thin surfaces and to m