A gravitational hook, attach to a small rope or cable, drop down on your target and grab it, the claw will close automatically using gravity.

Uses m3 screws, the holes are exactly 3mm so scaling the parts 5% might loosen everything up if needed.

This was made from an instructable, the author has made a kickstarter to have it made in metal, want one ? Go there :

https://www.kickstarter.com/projects/699604098/gravity-hook-a-high-performance-grappling-hook

Here is a video from the hook in action made by this gentleman : http://www.thingiverse.com/creuzerm/about https://www.youtube.com/watch?v=OfhQt22QKis