Quote Originally Posted by ecat View Post
In this case I think profile wins. k, back to the calcs...
Just to throw another spanner in the works, I'm planning on using square tube for best stiffness / cost ratio...

http://www.aluminiumwarehouse.co.uk/...duct_info.html

Not as convenient as profile but it should be possible to join quite accurately with a bit of drilling and tapping and a lot cheaper.