Off-the-shelf T-slot nuts are incredibly expensive. So many have set out to replace those with printed parts, always replicating how those nuts work thus putting a lot of pressure on the printed nut.
With this thing, all load of the joint is put on a 1mm washer, held in place together with the M5 half-nut.
This substitute prints without support and should be sliced at 0.2mm layer-height. If sliced at a different height, chances are it won't fit.
This part served as a T-Slot nut substitute for my MendelMax build, using http://uk.rs-online.com/web/p/products/837-234/ nuts and http://uk.rs-online.com/web/p/products/525-931/ washers together with KJN extrusions.
The OpenSCAD file contains part of Catarina Motas Shape Library: http://svn.makerbot.com/users/hoeken/openscad/shapes.scad
1) print many of them
2) insert nuts and washers
3) be happy that you saved a lot of money