All the dimensions given are specific to the form I built, any of them can be altered to fit your needs.
Step 1: Wireframe
Step 2: Tubes
Wireframe (original curves)
Inner pipes ( slightly large than the diameter of the dowel)
Outer pipes ( Inner tube + wall thickness of connector)
Joint Cutting Objects (set of spheres to make consisten connectors)
Dowel Cutting Objects ( optional layer)
Before you start designing your model mesure your dowels with a pair of calipers. It is a sad realization when you print out all your connectors to the size the size the dowel is listed as and the piece don't fit.
I used quart inch dowels with diameters that vary between 0.24"-0.25", so I used 0.26" as the inner connector diameter. Before building any pipes, make sure none of the curves are joined. The angles between curves causes interesting but troublsom irregularities in the connectors. Select all the curves, use the Pipe: Rounded Caps command, and then move all the pipes to the Inner pipes layer. Hide the layer.
I built the connectors with a wall thickness of 0.05", making my outer diameter 0.36". This is wall thickness should be adequate for most models, but might need to be adjusted depending on your situation. Repeat the same process as the inner pipes to create the pipes for the outer diameter.