Check out the Huey Thread stickied at the top of the Helicopter forum page. It has all sorts of pics showing it.
Here are a couple pics too.
Basically, the bungee was usually a thick piece of surgical rubber tubing attached to the carrying handle of the M60 on one end. It was then attached to the inside cabin roof on the other end. The attachment point was either tied or a ring was also used.
Hope that helps.
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