Parametric length arm for a RAM vehicular mounting system, which goes between two rubberized balls - http://en.wikipedia.org/wiki/Ram_Mounts
This item is ment to replace (functionally) the arm of a RAP-B-166-2U set - As such, it works on 1" diameter ball joints and the system is rated for support of 5 pounds or less.
Edit: Attributed Catarina Mota's hexagon function the way I should have from the beginning. Functionally, the file's the same.
Changed 4/12/14 to allow the option of a bolt/wingnut combo at each end instead of just one in the middle .
Changed 6/8/14 to offer a range of bolt size options.
Took off 'work in progress' tag 4/20/2016 - Lots of downloads and relatively little feedback - I'm going to call it done.
The file is only one half - Print one side with a nut trap, one without, use a bolt and a wing nut to join them as in the picture.
(Note: Pic is a M5-.8 x 50mm bolt and matching wing nut - The SCAD file supports bolts in the metric sizes M4,M5,M6 and M8 as well as SAE sizes 8,10 and 1/4" - I suggest using a bolt at least 40mm or 1 1/2" long . )
Strongly suggest using ABS if you are using this for a car dash - PLA versions to date have all warped badly and become unusable, but ABS prints have held up for me so far.