[sorry these are not the best pictures...]
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Print the file in nice colours.
screw in on top of the ultimaker exruder head
use the existing plunge or print a new one and use just the rubber
now the trick: use the filament to push the plunge. For better retraction light the filemant and melt it to the plunge
play a little with the settings [extruder speed must be a lot slower!] and start with simple shapes
if the filament grinds, detach it from the plunge, cut 7 cm and melt again

what i still need is an extra thing to attach to the sliding blocks that could hit the end stops. It could be something very simple clicked or screw which you can remove easy when printing 'normal' again.

It should work more or less the same on a makerbot i think. Let me know...!!