I changed dimensions of my 10-32 T-Nut to fit M3 hardware.
It requires M3 nuts, and I recommend supergluing each nut into place within the plastic piece to keep things from moving around too much.
Note that there is no overlap between the nut and the slots of the extrusion with this, so it won't be as rugged as the 10-32 version.
Oh yeah, and I made it customizable while I was at it, so you could recreate the 10-32 T-Nut with the proper settings.
If you're looking for something quick and easy that doesn't require glue and nuts, check this out:
Print with PLA or ABS.
Superglue an M3 nut into the hole.
Slide into the OpenBuild V-slot extrusion.
Screw your parts down!