Here on thingiverse is a scad for 3D printable shaft collars that will accept Hex Shafts.
http://www.thingiverse.com/thing:228005
We have also developed this scad for shaft couplers:
http://www.thingiverse.com/thing:229161
I would print this in Nylon however and on it's side as hex for strength.
The parametric wheel was used to create spacers.
http://www.thingiverse.com/thing:234935
Depending on how well your printer is calibrated I use 13-14mm for the shaft diameter to fit the 1/2" shaft snugly.