This is a bottle rocket launcher, to be used with a Gardena bottle cap. I used this one:
This thing requires a retaining clip bent out of 2mm wire, a car tyre pump and a board to mount it to.
I ran a drill through the clip holes to make sure they were nice and round so the clip was easy to pull out. The Block was then mounted to a sheet of MDF I had lying around. I also drilled 4 holes near the corners of the board so that I could peg it down to stop it moving when the clip was pulled out with a length of string.
The pump is attached to the lug on the side (it needs to stay attached during use as there's no valve in the base). The Gardena connector goes in the top and the retaining clip is pushed though the two small holes to retain the connector. I lubricated the connector with some Vaseline to improve air-tightness and to, well, lubricate it.
Early test assembly showing connector, pump and clip in place.
Close up with fins added to bottle. (Fin brackets: http://www.thingiverse.com/thing:2139616)