Well, since it's a fictional weapon, I think you can pretty much choose the style you want.  I've seen Dragonlance artwork depict it as a slingshot and as a sort of primitive Lacrosse stick.  The one I designed (that works better than my staff sling!) is a triangular leather pouch, tied to the staff at both ends of the forks and at the crux.  The cord at the crux is slightly shorter than the ones tied to the ends, so it stops the pouch from moving forward, letting missile continue between the top 2 cords.  If you do a search, I posted pics in here somewhere.