2x 15 !! You must be mad. Go over to talkaudio.co.uk, familiarise yourself with a router and a jigsaw and start crafting. A decent 12" 4th order bandpass will give you a nice boomy bass with reasonable punch (if well designed).
Surely the MK3 doesn't have the air volume in the boot to run 2x 15" with any level of efficiency, a single one, ported would fill the boot?