IT/Streamlight mount costs less than $15 usually.
It's a small black polymer piece about 2 inches long and you drill two holes through the lower heat shield in your handguards on the bottom towards the front just behind the front sight post (carbine anyway), run the screws through the mount and secure them in the backplate of mount.
It's for use with Streamlight's M3 series of lights but I imagine any light that would mount on a 3rd Gen Glock will work but I'd verify this as I am only assuming.
Scott