I've posted these pics before, but for a biconical training shot, are easy to make and give pretty good performance. Make three lengthwise cuts in a tennis ball, fold edges under and tie with twine or stitch with a ty-rap etc. Align in pouch so the cut edges are trailing the rotation for less drag.
Warning:
they increase the range of a regular tennis ball by quite a bit, and seem to make the projectile a little harder as well.
Additional Warning:
Your shot needs to be rotating with a reasonably good rifle spin and aligned on its axis with the target. If not, it will go more or less straight initially, but once the inertia drops off it will begin to follow the direction it is pointed rather than the direction it was going. It can shear off unpredictably. This is good for teaching, bad for shooting in a tight residential etc location. This led to a couple of "what the heck!" moments before I realized what was happening, shortly after which I lost all of them in the tall grass at the back of my neighbor's yard
I forgot to add, put a small notch at the midpoint of each "fin" to hold the string in place. Tied with jute, they are surprisingly durable - I never had to retie any of them, even after hitting trees, a concrete wall...