Car is a 1993 Nissan 240sx fastback, like the one pictured ...notice the angle of the hatch itself? I am looking to mount (1) RL-P15.

Here is my question: Would it be worth it to try and make a box with an angle that matches that of the hatchback glass, so that it fires directly/evenly into the hatch? Or do you think it would be better to just go facing straight up? I COULD fit the sub facing back except I REALLY don't want it to stick out above my back seat, which it would.....though if facing backward is that much better I am willing to sacrifice.

Also, is there much difference between 2 enclosures w/ the identical internal volume, but different dimensions?

Thanks in advance.

I would fire directly upwards and not worry about matching the angle of your hatch. In practice, a box placed in different locations within a single environment or the same box placed in a different environment can affect your realized frequency response. Therefore, using different dimentions for your enclosure even if keeping the same net volume and tuning (where applicable), can have an affect on your output. In most cases it will be difficult to estimate exactly how your enclosure will respond within its environment without actually trying it to experience the response first hand.