Did some quick modeling with winisd.
If you build a 3/4” enclosure to the dimensions you posted, it ends up at 5.1 ft^3 gross volume.
Add the driver, port, and bracing, and should end up in the 4.25-4.5 ft^3 net volume.
I modeled it with a single 4” port to save volume. Port velocity ends up a bit high in the low 20’s m/s, but I believe its worth it not to loose more volume.
The precision port length comes up as 17”, which is the uncut length making that simple.
Modeled with 500 watts and a 20hz 2nd order HPF looks pretty decent to me.
This design will require some eq, of course. Response is similar to the VBSS, but loosing on the low end due to reduced volume.
The sub goes in the 24x40 side, of course. I’m assuming that is the front. Due to port length, it will need to exit side or top. You could make it front exit by adding an elbow, but will require some custom work.
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