I am not a fan of prone or shooting off a bench when zeroing slugs. I like to shoot kneeling, supported by something like across the seat of an ATV. That provides a steady shooting position while letting your body move with the recoil.
Attributes that make a specific recoil pad more comfortable shooting slugs from the bench or prone may be disadvantageous shooting slugs for real. You could strap on a PAST shoulder pad or two on just for zeroing.