I know you don't want it in the trunk because of space, but what about this? Car Audio - Stealthbox® - Ford - Mustang - Ford Mustang Coupe '10-Up
It's only taking up the corner and is fairly low profile by design. Also it comes built specifically for the Mustang.
Even if this prevents your golf stuff from fitting in the trunk, would it fit if you fold the rear seats down? This way the golf clubs or whatever could mostly sit in the trunk, but also pass into the cabin a little bit.
I have the same enclosure but the 12" version. Doesn't really take up much space since it's tucked in. That space is pretty much dead space anyways! Still have plenty of room in my trunk.
2011 SG Mustang Premium V6, MCA Edition, 3:31 Gears, 6 MT, Reverse Camera.
Steeda CAI, BAMA Tuned SCT, GT Fascia, Boss/CS Valance, Boss Front Splitter, Saleen Grille, Roush Splitters, GT500 Rear Valance, Roush AB, Black GT500 Wheels (18x10 Rear/18x9 Front), GT Front Brakes, Eibach Sportlines, Eibach Adj LCAs, J&M Adj. Panhard Bar, Steeda STB, Grabber Pony SG Shift Knob, Custom Perforated/Striped leather shift boot and E-Brake Handle boot, SR Performance Shifter, Barton Shifter Bracket