If you’re an L-bracket user looking to use a holster as well, you know this isn’t an easy problem to solve.  If you put the holster pin or plate or button onto your L-bracket it takes valuable space from your clamp.  It also makes the camera unbalanced when you set it down on a flat surface.  Cotton Carrier actually sell the kirk clamp linked below for their application (but charge $59 for it -vs- $40 from B&H).  I’ve solved the problem by using the smallest kirk screw clamp they sell at 1″ wide.  I then replaced the thumb screw with a hex head screw for a lower profile and drilled and tapped a 1/4-20 hole on the other side.  You’ll only need to do the second part if you want to use a spider pin.  For the cotton carrier you thread their “button” into the bottom threaded hole on the clamp.  Here are the parts for this:

• one kirk QRC-1 QR clamp
• one 1/2″ 1/4-20 hex head screw (available at McMaster if you like)
• an arca compatible L-bracket for your camera
• the holster system of your choice (spider, cotton carrier, capture)