I have tried a few different connectors for the tubes. I made up some ball and socket joints like moonlight make for dobs. I use them on my 16" and they are great. I decided against it in the end because I didn' want plugs going in to the carbon tubes.
I felt safer following other designs that have truss adators which go voer the outside of the truss. Given that the carbon can be brittle I thought this would stress it less, being captive around the outside rather then a plug on the inside.
In fact this is what I am doing now, machining 16 of them.. I am sure at some point in your life you wish your lathe was CNC