Subwoofer selected:
Pioneer TS-SW2541D 10" shallow mount which only requires 0.35 ~ 0.7 cu ft. of air space! This enclosure is about 0.45 cu ft so its within
spec.
Amplifier selected:
MTX TC-3001. This amp was just the right
size to fit in the equipment compartment. It also has
speaker level inputs,
smart engage and a
remote bass level controller.
About fabricating the enclosure:
A cardboard 3D template was created and adjusted it to make the best use of the area available. Once the shape was determined, fabricating the actual enclosure was fairly easy. 3/4"
MDF was used, glued and screwed.
Testing the enclosure outside the X5 at loud volumes: There was no buzzing or rattling. Solid, like a rock! When mounted in the compartment, there is very minimal buzzing of the surrounding vehicle assemblies. The shape of the enclosure allows you take advantage of the OEM mounting points with simple brackets.
Specific wiring instructions: Read
How to add a sub amp to your factory audio.