Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:1739545.
Four identical parts go together to make a symmetric sliding mechanism. It is a difficult puzzle to put them together, and even more difficult to have them slide together so that they all have the same offset relative to each other.
This worked well printed vertically with no support.
This is joint work with Saul Schleimer.
Available from Shapeways at http://shpws.me/MCng