MT-2001 : Mantis Universal MagRail

Note: requires a flat magazine floor plate and bonds with high-strength VHB adhesive tape (included).  Yes, it holds up great under live fire conditions.  The stuff is quite remarkable.  And yes, you can remove it later if you want.  When installing it, you'll need to apply pressure, and then let it cure for 20 minutes before using.

Preferred if you don't have light-accommodating holster and want to train with holster draws.