How do I make a trebuchet?

I have to make a Trebuchet for a large medieval project, a large trebuchet actually, like this one:

I want to make my very own trebuchet.

I tried to follow this tutorial:

However, the tutorial does not explain how I can make the sling and pouch for the projectile follow the Rigid Bodies-rig.

Any ideas?

Should I use soft-body physics for the sling and pouch? How do I achieve this with Rigid Bodies?

I updated my post, I still have a problem making a trebuchet. Anyone with some ideas on how to animate a trebuchet with rigid bodies and soft body physics?