OxYoke or other woven felt wads can be bought in 16 bore size. Punch a 3/8" holes through the centres. Put down your hard card wad or wads, then 2 felt wads with the holes, then the ball, then thin card, then crimp.

The holes in the felt will keep the ball centred in the middle of the bore. Increasing or decreasing the charge might bring them closer to the rifle's point of impact.