15's in Servo_drive BT_7?

Toying around AGAIN with the idea of trying to retrofit a couple 15's into the BT-7. After bringing them back home from storage I am finding that a few of the motors are seized up. I never minded paying a bit for new belts, but motors are a new beast of expense I'm not sure I want to undertake.
Only problem is finding a 15 that is not too deep. You can see from the pics that if I was to take off the existing 15 inch cones, and tried to bolt on a 15 inch woofer in its place,......that would only leave me 5 1/4 inches/ 132ish mm clearance in depth. However,.......if I copy and rebuild the whole 'insert piece' and screw the 15 on the inside of the board instead,.....that would leave me 6 1/2- 170mm ish depth to play with.
Thoughts?