What I do is take a piece of dyneema line sold for spearfishing applications, which is 500+ test and very abrasive resistant. Dyneema is very resistant to moisture and UV. Tie a loop of the line through the hole in the butt of a gun. Tie it with a double fisherman's knot. Once tied the more tension you put on it the tighter it will get. Then just use a long line clip or snap swivel clip to attach to the loop of dyneema. Here is a link on the knot
The Double Fisherman's Bend