From everything we've talked about, we can assume we want the woofers firing into the listening environment. We wanna generate bass and energy. So we need to look for a driver that we can fire forwards. That tells me either 6.5" or 8" drivers. Now remember, before you start ruling out the 6.5", everyone usually jumps to the 8", that you can do three 6.5" subs in the same airspace one 8" requires and three 6.5" subs have more cone area then a single 10" woofer.
The next thing we need to consider is how much air space is available for us to work with. The OBS truck is quite the challenge when it comes to this part of a build. Personally, I would fiberglass the bottom of the box, but doing wood is also an option. You can just squeeze ALL the airspace when you fiberglass. I just don't know your skill set when it comes to fabrication.......