You might look at the TAD 1602. It isn't terribly expensive like their compression drivers are, and you could use them for a subwoofer.
Ft 21Hz
Vas 519L (18.3 cu ft)
Qts 0.21
Qms 2.78
Qes 0.23
Re 6.2 ohms
Le 0.9mH
Xmax 5mm (0.2")
Pe 300W
Sensitivity 97.5dB
BL 21
Impedance 8 ohms
Running PiAlign on this driver, we enter Vad=18.2, Frd=21 and Qd=5. It suggests a cabinet size of 3.66 cubic feet with a port of 3 1/2" x 6 3/16" and 4 13/16" long.