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.