My marble run system. OpenScad file is available. (See instructions.) My duplo-block-lib is an improved derivate from: http://www.thingiverse.com/thing:1778 Also see the marble pump add on: http://www.thingiverse.com/thing:350349I recommend to print one piece for calibration and if it does not fit well have a look at the variables in the top of the duplo-block-lib.scad file and adjust them as needed. The file duplo-marblerun.scad contains all parts as modules. In the top uncomment out the item to create. (In the OpenScad preferences you might have to increase the "Turn off rendering at ... elements" value, e.g. 1000000 or set a lower "quality"-variable.) I printed everything without support.