An Even Simpler Trigger Adaption:
Please understand, as in anything you do, there are certain inherent risks involved. Try this adapter at your own risk.
I have another even simpler trigger adaption that came about while I was working on something else. It starts with a cable tie, which are available in many different sizes. Pick a size that meets your purposes. Form a loop in the cable tie and slip the loop onto the trigger of your weapon. Draw the cable tie as tight as you can on the trigger. If you shoot right handed the cable should come off on the left side of the weapon and vice versa. Now you must get a method to assure the tie does not slip off the trigger when you apply pressure. This can be done in any number of ways. In the picture below, my wife has rapped tape around the bottom of the trigger. You could also use the rubber or plastic tip from a tool or the end of a bow piece from a pair of glasses. If you use a large cable tie the end of the head may be large enough to prevent it from coming off. If you wish you can place a piece of plastic tubing on the side of the grip portion of the stock and slide the cable part of the tie through it. When you are ready to fire simply get the end of the cable tie in your mouth, tighten it and move your head back to fire. This adaption work very well with Bog Gear Xtreem Shooting Rest (XSR) described in the Inexpensive Gun Mount since the trigger housing is in front of you and behind the rest.
Putting an adapter like this on your gun is not that difficult. What is difficult is to pay attention to the details so that you put it on correctly. All normal gun safety precautions should be followed. See PMTA for recomended procedure.