Making Stomp Rockets for Scout Family Camp (part1)

 At the risk of Homeland Security helicopters overhead, I've been making Stomp Rockets and a launcher for part of a science outreach activity. If you google or Youtube search 'Stomp Rockets' you'll find many articles and DIY guides.
I've gone with 22mm JG Speedfit pipe and twist-fittings and pipe inserts (From DIY stores like B&Q and Screwfix).
The launcher is made from a 4-ft, a 2ft and a pair of 1ft lengths (cut with junior hacksaw).. The 2 short lengths act as stabilisers having been lashed to the 4-foot section with a combo of rubber band, clingfilm and garden twine (plus some sellotape!).
For the tricky job of mating the 2L empty drinks bottle (the stomp) to the pipe I'm experimenting with two approaches.
1) a bottle cap with a hole cut into it (using some v. sharp scissors rotated repeatedly into the little central dimple on the cap) that just allows a pipe insert to go through and  2) a 'fruit-shoot' type bottle cap (NASA are you jealous yet?)
with the white safety cap cut off (the scissors again) and the central insert cut out (little blue 'pip' in photo)to allow air through which then, with a bit of judicious scissor shaving, goes into the pipe insert. The goal is to have easier replacement of bottles after they are wrecked by stomping (Well easier than messes of gaffer tape...).
The bottom 2 photos are of completed launcher (OK it's no Falcon 9) which now needs some testing...