V6GOOSE:
I actually used carbon to carbon with enought medium cyano glue, and it becomes harder than metal.
In fact i just drilled a 1/8" hole at the top of the mast and cyanoed the 1/8" tube for the crane. it never moved!!.
try to get a carbon tube wich its inner diameter fits another Outside diameter in a carbon tube. most of the Us stores has many carbon tube sizes, but sometimes they do not have diameters that allow the "telescopic2 effect.
i found carbon spars at
http://www.r2hobbies.com/products.php?cat=136 and they are telescopic, it means the smaller fits the inner dia of the bigger. just use a 4" inner tube as a ferrel and you are done!!!
Hope this can help, prices looks great too.
Tato Lazo
CHI 272