My idea of best bang for your buck: 2 Polk audio dxi104s. Theyre on sale at crutchfield and has got scintillating reviews! Clean crip bass.
For the box, google custom subwoofer box for 2013 mustang
. There are couple websites who makes them if you wanna save trunk space.
Ps: since you have shaker system, I doubt if your subs will run at their full potential. Reason being you'd have get the output signal from your door subs to be able to use low frequency signal.