For the other end of the oxygen tube, we inserted a bog-standard bicycle valve into the bung, and fixed it with rubber glue:
Here's how we attached the bung to the oxygen tube - with steel wire. Note that we rejected our first idea of using worm drive hose clips, because they wouldn't fit inside the PVC tube.
Here's the same passing through the PVC end cap. This differs slightly from our concept graphic, which shows the entire bung protruding from the PVC tube:
I know you've probably planned everything already but...
...coil the tube up into a spring shape for space savings? It'll still expand inside whatever drum or coiled tube you put it in. Also I'll second Parax; that PVC tube can probably have more than 50% of its mass drilled out and still retain enough shape and strength to function. That's if you don't decide to go posh and use a glass or carbon fibre tube (or old fishing rod).
If you want something super-light and you think you can form it around something (dowel and some grease paper possibly), model shops will sell a kit of thin glass fibre sheets and resin that's designed for constructing helicopter bodies and suchlike. It'd take a bit of work and sandpaper, but you would end up with a very lightweight fibreglass tube that can be as thin as its task allows. It probably wouldn't blow the budget either - I think I remember it being a tenner or so for about 4 1m/sq sheets plus bottles of thin adhesive/hardener.
Last but not least, I hope you can get a couple of nice telescopes to film this thing through. That or let any commentards know where and when with enough notice that they can bring theirs. This is daftness on a scale of painting a Robin Reliant up as a radio-controlled space shuttle and sending it heavenward on the most powerful non-commercial rocket built in Europe. It'd be a shame to not have a video of the fun.
So if the tube is only for structural purposes and not to act as a pressure vessle why not get the big drill out and perforate it? should save many grams doing that. Additonally why not use a carbon/glass fibre push rod? An Angleing store or a Kite shop will have what you need.
Whilst others have tinkered only one really succeded. A cradle is for nuturing growth after the birth not for conception.
oh and perhaps you mean escapement not catchment.
Education these days really is piss poor. (as I beleive the OP was suggesting) Threading a brass rod by hand would only take 10 minutes but they don't sell Taps and Dies in B&Q!