That's a problem with isobaric reflex alignments, the porting area requirements stay the same, yet the box size is cut in half. This tends to make port lengths silly long.
I know "car subs" are normally a not go for home use, but what about "Kappa Perfect"? it is somehow cheap now, its pretty (to use in an isobaric sub), decent Xmax and works in small enclosures...
Iso's can be Ported, Sealed, and PR'ed, tho venting can be hard to fit. Drivers selectd for use and ultimatly the end box size will be the limiting factor aside from the extra costs involved.
You might check out this site and at the very bottom, for some basic loading ideas and the plus and minus of doing so.
Heres a Iso front to back, or cone to magnet rig. It's using a set of RF-12's tho info on it's design and or performance, well just ain't their.. I first assumed it's for a car, but might be for the house seeing that it's veneered. Also in a larger than usually needed cabinet for a car/truck with their mucho free gain.
Interesting way he set up access to both drivers while allowing for a good seal. Nice to know Wax Paper (dose not seem) to stay stuck while the silcone seal drys. Something I did not know and others might find useful in a up-coming or on going project..
A sealed design with isobaric subs might work better, though, to keep the woofers from bottoming out. My first sub was a crudely-made design with dial 8" isobaric woofers, but the bottoming could have had something to do with the fact that the tuning point was way off. I scrapped that one in a year. I don't even have the woofers (pioneer 8" DVC) anymore, or I think I could have still used them in a new design.
Edit: how about a quad-shiva isobaric sealed design in a sonotube? There could be a clamshell at each end of the tube.
I asked about exactly this not very long ago, wondering if it would actually quarter the required volume...but if you think about it, it would really be a dipole. Would make for an interesting experiment though.
On a side note: I rejected the Shiva for isobaric configs. Can't remember why. :frowning: