Okay, someone had to bring this one up...
Seems a bit cheesey, but my first ball bag was made from the cover of an old softball. My son and I hit that ball so many times that the cover was loose. I cut the rest of the stitching and looked at it. I folded one half over onto itself and thought it made a handy ball bag pattern if I cut the connecting piece. I sewed it up and made a bag for him to use in his YHEC shoots.
I have made several more over the years from that pattern. On later versions I also used a bit of copper tubing which I flared at each end to make a rigid end to pour shot out of.
I used a wooden dowel to whittle and file a stopper which I secured with a leather thong. Draw out your pattern onto the wood, place the wood in a vise and rasp it into shape. Use a thin saw to split the part that plugs the tube and use a wedge to adjust for tightness.
Worked to contain anything from .50 cal to #8 shot.
I guess the big bore guys could just use bigger tubing.
YMHS,
CrackStock