I suggested 4" tube, but if you look around you may
find the remains of broken carbon spinnaker poles of
the standard 3.5" size for cheap. They also sometimes
come with the fitting, making gooseneck fab a bit easier.

Another common source for carbon spars is broken Melges
24 masts. The original masts were weak at the spreaders
and many, many M24 owners have the pieces laying around
in their back yards. Great for makeing strong booms out
of.

Wouters design is pretty cool. One other thing lazy
designers do (besides use carbon fiber) is sometimes
they design elegant creations without regard to the
labor involved. They sometimes add 20 hours of labor
to save $300.00 in materials, those pesky critters do.
His design looks like fun to build for the hobbiest,
though, and I bet it works.