Summary

Parts designed to use multiple lengths of 1/2 inch Schedule 40 PVC pipe for greater strength and rigidity.
.
Also may be used with wooden dowels (of appropriate size) if the use of PVC offends your sensibilities.

Instructions

ABS recommended; scale to fit the PVC pipe available in your area.
.
Once parts are screwed into the pipes the objects are surprisingly strong. Could be useful for creating machine mock-ups before committing to cut aluminum extrusion.
.
Sample images show painted parts to provide better contrast (I used natural ABS which didn't photograph well).
.
The black part is from the original PVC pipe construction set.
.
Designed with TinkerCad.

UPDATE: 21-Jul-15: A few parts require supports; be very careful...there are many pointy bits and you will bleed.
.
A ratchet-style PVC pipe cutter is a good investment if you like clean cuts on the pipe. A chamfer on the ends will allow easier assembly.
.
Because of layer orientation, some parts may separate if the fit is too snug. Sandpaper is your friend.
.
Updated files with simpler screw locations will be forthcoming.

UPDATE 31-Jul-15: The parts are attached with small sheet metal screws; I drilled 2mm pilot holes for them.
.
UPDATE 10-Aug-15: Added part ("T-Bone") to allow simple t-junction.

PVC pipe construction set, ver.2