For smaller catapults, I like making slings out of duct tape- it’s easy to make and relatively strong. But they are prone to tearing and can be bulky (especially after being repaired). This was a problem with Drill Sergeant, but I think I’ve come up with a good solution.
A combination of Tuck Tape (a strong polypropylene sheathing tape) and duct tape makes for a very strong, lightweight and flexible sling material. Wrapping the Tuck Tape sticky-side-out around a piece of drain pipe gives the perfect shape / size, which is then covered with duct tape (sticky side in).
A small wire frame is then taped on. The sling lines tie onto the frame. That way, if the sling needs to be replaced, all you need to do is cut the tape off the wire frame and tape on a new one. Even if the new sling had to be made from scratch, the whole replacement process would take less than ten minutes- useful in a competition scenario!