The "box" one will still be pretty tall at 30-32" for the "box" and since I am mimicing the sound of the SVS, it will be down-firing so add 3-4 inches to a base for it. I figure it is getting hidden, I need to replicate 4 cubic feet, and it still needs to be skinny enough to fit behind my false wall. It will have a top port on it (just like the SVS) with an almost flush grill (don't want them to touch and vibrate).

Good call on just doubling the diver baffle. Makes sense and will really make it solid all around. With a single sheet of MDF (49" x 97") I should have enough even with making some braces internally.
2-M60s, VP180, 8-M3s, SVS 20-39PCi, DIY Sub, 8-Shakers, JVC RS45, Anthem MRX-1120