Gonna follow the KISS principle. Plus, my local hardware store only sells 10' pieces. The 3/4" seems to bend just fine for me, and I think I can get away with 1/2" on the spine, since there's no load on it - I have yet to test it under strong wind conditions, but I have no reason yet to be pessimistic. I only hope I can find time to test it before it starts to snow.
I do like the way that electrical piping fits together, much easier than fucking about with water piping, since there's no requirement for it to resist high pressure. The only problem is that I can't find X-joints for the electrical piping, so I have to fit two T-joints together at double cost
When you factor in the time saved, I guess it evens out.
Thanks everyone for your help, I think I've got this sorted out.