I've
never closed the bottom of my gig bag since I've owned it. The rubber-tipped endpin, which is at the bottom of my B2 prototype, has always protruded from the bottom of the gig bag. (end quote)
That is the design. So the short zipper comes from the back side, and the long zipper comes from the opening side, and you have the two zipper pulls on either side of the endpin hub, right?
The chancy part is when both zipper pulls migrate up a bit and the opening enlarges. Hooking the two pulls together prevents that.
The bonus is that when folks see you have a carabiner, they immediately think you've done Everest or K2 and they give you more respect. If they don't, whack 'em with a crampon. It's always worked for me.
Lee