Product review:
I installed two 4-ohm Tang Band subwoofers to replace the blown dual-voice-coil Bose sub in my Z3. I made a new litttle sealed box for them that fit in the stock location between the seats. I think they were this model:
Tang Band W5-1138SM 5-1/4" Neodymium Subwoofer | 264-831
I was very impressed with the quantity and quality of bass these little subs could put out!
Tang Band has 5.25" and 6.5" subwoofers (including other small sizes), so you might be able to find something that would work.