Well I had it open because it would be enough internal volume for the subs, I know there is something you can put inside the box to help with the internal volume but I cant remember what it is
The baffle between the two subs is VERY IMPORTANT and HAS to be their, the white stuff your talking about is only used if the box is a little too big and you want to bring the internal volume down in size and doesnt have to be used. Its also used for fibregalss enclosures which can sound flat at times the fill helps with the "solid" sound.
i was lead to belive that this is the other way around, you use a dmapening agent if your box is too small, it will give the sub the impression the box is bigger by delaying the air from bouncing of the rear wall/s of the enclosure, i used to use a simple block of timber to bring the volume of the box down.
the centre baffle is important, it will sound crap and lessen the lifespan of the subs, it also pays to hav either the sub or the rear of the enclosure on an angle, the air moved from the sub will bounce around in the enclosure rather than just hitting the back of the enclosure and then bouncing straight back into the rear of the sub (there is a lengthy scientific version but this is easeir to understand, even a simple abgled baffle in the box will siffice.
good luck with the system