Again id ike to change it a bit converting to a trapezoid.
I modelled it with two drivers, and the dimensions of the box you sent me. So that gets f3 to 98.65 Hz. So not 85Hz. Now this is a model and not measured. However my experience with this program is that impedance is measured and modelled are in excellent agreement.
Now I did play with the box size, and those dimensions do give the lowest f3. If you increase the box size, f3 starts to rise fast. However the downside of that box is that it runs out of xmax with very little power at 110 Hz.
I will be honest, that is not a good design. Sealed boxes are highly inefficient in the bass. If you build that it really needs to crossed to the sub at around 120 Hz.
That design is just not a good practical design. If I had done it, I would keep very quiet about it!
Changing the shape of the cabinet to trapezoid will not matter.
Those drivers would be much better in a ported design. A 1.2 cu. ft cabinet, with a port 3" flared at both ends, and 3" in diameter and 6.2" long gives you an f3 of 48 Hz, and power handling of 55 watts at 80 Hz and common crossover point to subs.
That will be a much better speaker, more useful and practical. That Richie bloke needs a few lessons.