The factory passive radiator, although capable of very long excursions, is for some reason tuned too low to work in a monitor-sized cabinet. That's even before any weight is added. And as Jim pointed out, you need a longer port than is practical in a monitor. SB Acoustics has an oval passive radiator that will work, but it's X-max limited. Plus, you really need two of them, which probably means mounting them on the sides for a weird look. Purifi makes a 4.5" midwoofer that might make for a super mini speaker, but it's not usable in a passive design due to extremely low sensitivity.