ineinandergreifend d123|17
This simple interlocking form printed with TPU allows to attach parts firmly still be able to easy remove them. For better demonstration and first test i only used an extruded 2D shape but if altering the shape along the Z-axis you will also get the 3rd dimension locked.
Satisfying to watch 
totally, i should make a gif …
kind of mesmerizing, here you go…
missing/deleted image from Google+
And from the area of more rigid materials I have found very useful these Snap Pins: http://www.thingiverse.com/thing:213310 (very useful for positioning before glueing etc.)
http://www.thingiverse.com/thing:213310