Driver Location in Speaker Box?

I like to offset the cutout in a box that size if its possible for the situation.  It makes it easier to build in a crossbrace at the center of the box.  Like the following illustration but using only one sub instead of two:

In your box, the center areas of the front and rear baffles are the points most likely to flex.  A brace from front to rear as close to center as you can get it is in order.  The full structure brace shown above accomplishes the same task as I described.  Even though front to rear is braced at the extreme top and bottom, the braces are joined with a rigid vertical member on both ends, forming the equivalent of a complete side baffle.  You get the same results if you were to join front to back with a single rigid piece of wood placed at the center of the box.