I designed a 3D printed sling for MlePh's trebuchet model. It uses the same dimensions shown in the original model's PDF guide, but with extra tabs for 3mm holes for tying the string instead of sewing it into the sling.

This must be printed with a flexible filament such as TPU or FPLA. The two holes on each end of the sling should overlap, so you should tie one of the string through both holes to pull them together. This will force the print into a sling shape instead of remaining flat.