Irregular Enclosure Shapes and Sound Q

I have read a number of posts on the forum that state that the shape of an sealed enclosure is not really a factory in sound quality, and I have a question relating to that. 

I am considering building a fairly odd enclosure to fit a depression in the rear of my regular cab silverado.  The portion of the enclosure that houses the driver will be a somewhat conventional wedge, but there will be a part of the enclosure that goes up and over the transmission/exhaust/gas tank hump that would be fairly narrow. 

The wedge portion if the enclosure would be approx 12" tall, 6"deep at the top and 8" deep at the bottom, this portion will be around 12" wide.  The part I am mentioning will be in the neighborhood of 6"x4" and 20" or so long extending out of the top of the wedge, length would be determined by how much volume I need to make up.   Materials will be 3/4" MDF and Fiberglass, so the internal dimensions of this extension could be as small as 5"x3"

Is having a long and narrow section in my enclosure like what I am mentioning effective?  I am trying to keep this thing as low profile as possible.  Or is there something I should know?

One other thing, if no one sees a problem with what I am mentioning.  How small could an extension like this be before it becomes a problem?