Here are some tried-and-true designs that may be a help in case you haven't found your favourite slingshot shape yet. All templates can be downloaded and used according to the notes.
Free to use for: Commercial, non-commercial use
Download: Partymix1.pdf, Partymix2.pdf
A bunch of tried-and-true classic slingshot templates.
Author: Bill Herriman (Tex-Shooter)
Free to use for: Non-commercial use
Download: Tex-shooter Classic
The "Tex-Shooter Classic" is a great slingshot. Few designs have been copied or modified as often as this one. Its simple shape makes it usable for almost everybody, its wide fork prevents fork hits and the clever use of a Forstner Drill allows you to produce the slingshot with minimal effort. I hold this design in high regard, and wholeheartedly recommend it to beginners and experts alike.