I was inspired by http://www.thingiverse.com/thing:35564 but i found it a bit ...too big, with useless use of plastic
so I design this one to self centered, with just the amount of plastic to be strong enough to keep the nuts and screws in place

designed for 5mm stepper axe and 8 mm threaded rod

with inside guide for the axe and rod to be perfectly centered

need 6 3mm 8-10 mm screws and 6 nuts each (on the picture I had only 16 mm screws)

I printed mine 0.2mm high, 0.49mm wide 0.6 infill in ABS (i found PLA a bit brittle for this kind of job)