1991 mustang sub box .

I can't imagine more than 2 Lanzar 15s in the back of a Mustang. Needs too much airspace. The design also calls for them to be in a box of some sort. They were not meant for "free air" or "infinite baffle" setups.

You can do what you like so long as you can contain the subs in a box. As for direction of the subs facing the rear windshield, Not an issue so long as they have some space above them to breath the voice properly. I recommend they have at least an average of 7 inches of space between them and the window.

I would get a box that can fit the subs and in the needed area. Then get another board as a beauty panel to fill in the area around it to give it a cleaner appearance. Now you will have controlled sound and a clean(er) finish.

My guess is that you are not looking to go to competition with these are you? If I am assuming correctly, then you'll be fine in getting a sealed box to hold the subs and control them. Ported subs will be bigger but give you bigger boom.

You can get a similar boomy sound too from the sealed by filling them with about 1lb of polyfil (pillow stuffing) for every cubic foot of air inside the box. This will give the boomy sound but not the extra loudness (or size to go with it).

Ganbatte ne!