A fully customizable coupler, based on my previous one : http://www.thingiverse.com/thing:286339
I hope you'll enjoy it!

Edit: I updated the y position to enforce it a bit, and changed the nut default size to 6.25mm, because 5.5 was too tight.

Well, you just have to define your axis diameters. The widest diameter should be on top (unless you like adding support material for nothing ;).

The nut diameter is the maximum diameter of the nut (distance between the edges, not between the faces).

I think it's all you need to know, the rest is pretty straightforward!