SB Acoustics SB34NRX-00 12″ Coated Paper Cone Passive Radiator
This passive radiator is a good match to the SB Acoustics woofer SB34SWNRX-S75. In 2.0 to 2.5 cubic foot box this combo will get down to 30Hz. Adding a plate amplifier with a low frequency boost would push the F3 even lower.
Spec sheet: SB Acoustics SB34NRX-00 12″ Passive Radiator