There should be printing companies around your area hommie, remember they print banners, well, for your information, banner rolls coome inside of a cardboard tube wich is 8" wide, they work AWESOME, and best of all, they are free!!!! Just go to a printing company and ask them to give you the cardboard tube, they always throw them away
Here you have an example:
And a box a friend made with them also, i have built a few with them but i don´t have any pic, best of all, they are easy as hell to flare
P.S: I still think that box is too big, i wouldn´t do more than 2-2.2 per sub