Gravity Grabby is a grapple type device i designed a few years ago. It's basically a simple pivot point that opens up the teeth when it's dropped on something. Then pulling up closes them, hopefully grabbing the object you are trying to get. I have mine on a small carabiner so it can be attached to string or anything you can tie around it, but you can just pass string through the top holes or ease.

It requires 3 x M3 x 14 screws and 3 x Nuts, i put some thread lock on them as you find they can work loose. Nylocks would be perfect but i found they made the side profile bigger, and it been slim means it perfect for fishing small things and cables from the back of the sofa.