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.
2012 Mustang GT Premium (Manual), Sterling Gray, Brembo Package, 3.73 gears, Electronics Package, Comfort Package, HIDs, backup camera, Koni Yellows, Steeda Sport springs, Steeda HD mounts, UMI panhard bar, Borla S-Type catbacks, MGW shifter, Driveshaft Shop carbon fiber driveshaft, RTR wheels, Michelin Pilot Super Sports, Roush front fascia, DG Custom hidden fog billet grille, Roush lower billet grille, Roush front and side splitters, DG Custom quarter window louvers, GT500 rear diffuser