cyliner enclosure

Does anyone know how much flexibility there is in port length and width, so long as the volume is the same?

That statement is what is wrong.  "So long as the volume is the same."  That's where a lot, and I mean a lot, of people mess up on getting the concept of ports.  It's not the volume of air, but a much more intricate calculation involving air mass, outlet size and speed of sound.  I don't get the equations, but I get the principle of it (and thank you Lord there are calculators!).  The air mass in the port is important, but only as important as the diameter of the port which is what allows the air to escape at a particular speed.  It is the combination of port diameter and port length that is important....not the cubic inches of air in the port cylinder.  A port can be cylindrical or square, you take your pick, the principle remains the same.  Choose port opening based on the overall look you want and the best build design for the system. 

Porting through the seat will do nothing for you.  But if the subs are driven into the cabin area, port into the same area.  If the subs are driven in the trunk, port into the trunk as well.

The enclosure, too, can be of any shape.  A cylinder is inherently strong with little chance of flexing.  The difficulty is in the construction.