While I think the original Printrbot Z axis shaft coupler is clever, I don't really like it in practice. Mine periodically slipped on the shaft and ruined my prints.   

 As you can see the finished product worked well enough to use on my Printrbot. But it is worth noting that there is no vertical support when you use these, you can just grab the printer's z axis and lift the entire thing straight up any time. So you have to be careful to make sure you don't disturb your printer too much while in use.  

 This design can take a lot of misalignment and it wont slip, but it is also taller than the original Printrbot Z coupler.  

To be honest if you are willing to part with maybe $20 I would just buy some aluminum bellows, disc type, or slit type flexible couplings because those lock everything in place and you cant accidentally lift the Z axis up.

Aluminum version on amazon here: http://amzn.to/28Ozw0z