The OP is limited with volume and especially depth behind the woofer. Isobarik is not exactly a volume saver option.
True. A tall, narrow, and somewhat deep cabinet is theoretically a candidate for a vented box, as the cabinet height is reasonably likely to allow adequate port length, which is not always easy to shoe-horn into a subwoofer cabinet.
How about using a pair of KEF B-139. Several used on ebay <$100/pair. Or better use 4 with one each out phase per:
Need basic advice to make a pair of magnet to cone configuration Isobarik sealed chamber bass woofers. I am confident with making the cabinets just not the technical side. I have 4 x B139 SP1044 2 Kef Concerto Falcon Crossover Dn12 Upgrade. Can I connect 2 bass units to one bass...
www.diyaudio.com
Or New:
https://www.falconacoustics.co.uk/falcon-b139-8-ohm-kef-b139-sp1044-replacement-woofer.html
Wow that brings back memories. First pair of speakers I built in 1979 used KEF B139 SP1044 woofers in transmission lines. Back in those day the cool kids were into transmission lines, which did some things well and other things not so well, at least at my amateur level.
I subsequently put them in an isobaric subwoofer, and found out the hard way that as the inner woofer increases the effective box size seen by the outer woofer, the outer woofer becomes susceptible to over-excursion and snapping the voice coil against the backplate. Sounded like a machine gun, gave me a full-on adrenalin dump, and time slowed down while I lunged for the volume control knob. (Kept that little fiasco to myself to protect my imaginary "cool kid" status, but I'm no longer cool so there you go). The B139's one-way Xmax was about 4 mm, and its one-way Xlim (damage limited excursion, which I ran up against) was only about 6 mm. Those figures were probably not bad at the time, but would be considered abysmal today. Presumably Linn used a custom long-excursion version of the B139 in their big Isorarik speakers.