I don't have time to post my elaborate summary but here's some cool files. The mesh is generated properly, and I have tested them to fit. Have fun.
Think of this as a sample. The STL you should print is:
This size good for flashlights etc. It is a sample. I have also included the 10 slot version that fits my pressure switch on MPX. SIG finally sent me it... ~7/2016-4/15/2017 almost a year nbd
NOTE: Keymod uses M4x10 (#8-32x1/2"), Mlok appears to use M5x10 (#10 bolt)
P.S. Thanks user Mondo from GrabCAD for .SLDPRT for extra certainty when modeling.
P.P.S. /u/Repstar your license does not allow me to Remix and thus properly credit you
Edit: Note: this may not be compatible with official Keymod hardware. I used an M4x10 because that's what I had lying around. But, this should work within the accuracy of your printer.
Also, this is in no way my endorsement of Keymod, vs Mlok, I just finally got my long awaited Sig handguard and wanted some parts because I only have Mlok parts. There are a few reasons why I think Mlok will serve us better in the long run, especially for making 3D printed parts, but it may vary based on application. Still nice to explore the other system using only a couple bucks worth of parts.