If you want to attach a leather strap onto your main stock to give you more leverage and dexterity, one problem is that it always wants to slip away down your arm because both ends are attached to the main stock. You can avoid this problem by attaching a metal shackle to the top end of the rear of the bag like this one:
http://www.toolstation.com/images/library/stock/webbig/97943.jpg
You take the bolt out and find a smaller bolt or flat pointed screw and insert it through the threaded end (in the opposite direction – in the picture, coming from the left side inward). In order to avoid any bag tears, be sure to reinforce the area of attachment with a piece of thick leather (I even added a small piece of copper [edges sanded dull]) glued inside the leather, just to be on the safe side). Then, you can tie the other end of your strap to the metal loop (the other end being tied to the main stock).
