I picked up a Trufire bow release and modified it to grip a 3/8 inch pouched ball.
I removed and discarded the wrist strap and replaced it with a loop of paracord (that I hook around my ring finger), machined off some bits, and made a couple of stainless loops, beveled the inside edges, and silver soldered them on.
I mounted it and a slingshot on a piece of plywood and after I dialed it in (I found that it likes a thin pouch, and ball placement in the pouch is pretty critical) the balls started going in pretty much the same place.
I may make another one with bars (like the Dankung release) instead of loops.
Here are a few picutures.
A few shots of the modifications:
10 Meters to the target over my plywood "shooting fixture". The red line is not a laser; rather, it is braided red rubber bands at 32 inch draw.
The target after about 50 shots from the shooting fixture.
A close up of one of my exhaust pipe slingshots (I posted about them on the Homemade Slingshots Forum a few days ago)