If you plan on putting high power subs in it, maybe you can brace the inside a bit, since its a long box (or high, depending how you look at it)
I would use broomsticks and glue them inside, from one panel to the opposite panel, where you would normally have put the extra wood if you would not have let the woofer share the same air.
hmm, English sentence not good here.

Let me know if I should rephrase that.