This design has found a new home...
(This applies to the yellow part in the pictures. In the black and yellow assembly you see it in use)
This is an adapter plate that will allow you to put a NEMA 17 compliant stepper motor where there's room for a NEMA 23, and vice versa. I've tested it to work fine for putting a NEMA 17 motor on my X-motor bracket.
The SCAD has become a bit messy in the process because I should have used the specification instead of trial and error. But you will probably only need the STL anyway. The rendering looks odd, but it slices fine with skeinforge.