I dont know about fitting square 15" kickers their unless you plan to use the back seat lol. But TC customs has really nice custom fiberglass boxes and they have a custom mustang box for 2 12" that holds speakers 7.5" deep. I have one and its solid I added some spacer rings to give me extra inch added 2 JL W6v2 so you can have decently big speakers in a built box. Mustangs are pain in the A** for trunk space and mine the box went in first then the speakers ***But I removed the sound board in the back and this opened up alot more room with it in their your SOL on space dude hope this helps.
