****Although I am not familiar with the size of the Glock 21 - Fobus makes a workable pistol/laser holster for the Glock 17/22/31 and for the 19/23/32.
The one for the 19 size frame (I have a Glock 19.. with laser combo) uses a nice leather thumb strap retention mechanism that allows fast presentation and secure positioning of the gun. It is a roto-type and has a great degree of adjustability. Pic below...
Any of the well known kydex makers make custom stuff all the time. You might need to mail the light to them but the can make anything. I have a bravo concealment and I love it.