Some will definitely argue with this but it's been documented that the sprue shlould
not be a concern. There have been posts in this forum in the past. As a rule or practice, you center the sprue pointing up. .....
I run my own lead in the summer, have shot both side by side and have seen no difference between them and the factory balls.
Now then, if you feel that it does, I'd suggest and old cast iron frying pan and let them roll around in that. Would save you from having to kill a full grown plywood. ....
Be Safe !!!