Do not go to a craft store for stuffing, they'll sell you basic pillow stuffing/ blanket/etc.
That is not fire retardant, subs get HOT, and will light stuffing.
Do some research, you'll figure it out. And as everyone else said Sealed box per sub.
Use 1lb of fire retardant poly fill for each square foot you need to "add". Don't overstuff the box, leave room for sub cooling/ air to move.