This is a remix of the ergonomic slingshot design by sylvar75.

First I used MeshMixer to lengthen the forks because I thought they were too close to your hand.
Then I used TinkerCAD to add a hole in the handle for a lanyard, and I added holes & slits in the forks to make it easier to attach the rubber bands.

The pouch is made from layered gorilla tape.

The print shown is scaled to %95 size. If I did it again I would probably scale even more to 90%.

Contrary to most people's gut reaction, you don't have to print this solid. Mine is hard as a rock printed with %25 infill and 3.2mm walls in ABS. You could go even less in PLA.