When you make a ball holder for larger calibers, weight becomes an issue. That thing is often hung around your neck for convenience, particularly if you are doing a Seneca Run or some other speed contest. It does not make a lot of sense to have just ONE ball black, holding 15 or 20 ball of .58 caliber hanging around your neck all the time. Its better to make smaller blocks, holding 3-5 balls, and just make several of them for those rare times when you need to have all those balls to run a match course. In the field, You are likely never to have a situation where you fire more than 4 balls ( I in the barrel and 3 in a block) before you get back to camp, or your vehicle to replenish your gun, and block. The huge blocks with lots of balls in them begin to look like a Fraternity paddle, for the Marquis de Sade!
If you have access to a router, you can cut grooves on the underside of your block that will match the width of your barrel flats, and that can aid you in centering the ball over the muzzle for quick insertion, using a short starter, or if you are very practiced, just using your ramrod.
I have a block for my .62 smoothie, that hold 4 balls( 3/4 ox. each!) , and I definitely notice the weight around my neck. I find myself leaving one of the holes empty these days. I may just make a smaller one.