Breaker bits (jackhammer ) are really easy to find, at tool rental places, if they are too short they discard them. They have a nice collar already there and can be easily forged from the five sided shape to a hardy hole shape. Forge it square and parallel, then hot filing it to the final fit. Keep it square and not tapered then the hardy does not wiggle, for the cutting end.. let it be rounded, it cuts well that shape, and there is less chance of nicking the hammer than if it were flat across. Lot less work than altering a trailer ball.