Matt: When I wire up a beaver sack, I use a separate piece of wire to tie the sack shut. Here's how I do it. First, I twist the neck of the sack shut. Then, I spear about a twelve inch piece of wire through the sack material just below the twist. Then, I wrap the wire around the twisted neck of the sack, and finally twist the ends of the wire together. Spearing the wire once through the sack keeps the wire from sliding up on the sack and possibly slipping off. Twisting the wire around the neck, obviously, keeps the sack closed. To finish rigging the sack, I take the drowning wire and fasten it through one or two loops of the wire already twisted around the sack. This makes a hinge joint at the sack, and the drowning wire is less likely to break fastened like this instead of directly to the sack.
-- Hal