The GlockMeister part, an original, has a protrusion that snaps in to the lanyard hole in the back strap. Unless it breaks, it's not going to slip, it takes a lot of force to depress that piece when removing.
I would recommend a GlockMeister plug. IMO: It's one of the best flush fitting plug on the market. If you call them, you can get $2 shipping for orders less than $20.