Two things: This speaker is heavy and I do not have a large photo studio, hence some white sheets and two lamps in my workshop. I hope it provides an impression of the sculptural beauty of Jesper's latest creation, the Ekta Grande.
This construction will be rather quick and dirty. Most of what can be said about the SS drivers can be found in the Ekta and SP38 files. The basic cabinet coordinates can be found below and to make the crossover work properly, the front panel dimensions and driver placement must be kept fairly accurate. What's behind the drivers is up to you.

The bass cabinet is 65 litres and this speaker goes low, really low. The mid volume is 5 litres, quite generous and will allow all possible damping experiments. Actually you can reduce volume for the bass drivers by sacrificing a bit of low-end extension. 55 litres will still provide a powerful bass I'm sure.

Two tweeter options will be given, no more, so please do not ask. The 7000 and XT25TG are so alike that only minor changes to the crossover is needed and the exceptional linear response from both tweeters on this baffle allows very simple crossovers.
Initially a 3½-way crossover, quite similar to the Ekta, was constructed. 4th order to the tweeter and the lower bass working like a semi-sub. This made an even more heavy low-end and all in all it sounded quite nice. Flat response and a balanced presentation of all kinds of music. Playing around with the LspCAD and looking at the almost ideal response of all drivers, the idea of making a minimal crossover was obvious. SPL of individual drivers seen below. Making a minimal crossover would also allow the use of some of the best components available.

The crossover is 4 coils, 4-5 caps and 5-6 resistors, depending on whether the 7000 or the XT25TG tweeter is used. I like the XT25TG version the best. Really can't tell you why, but difference in the coating of the two diaphragms may be the reason. The 7000 has a thinner coating, probably providing some of the extra sensitivity and probably reaching a bit higher in frequency response. I can't tell, because the CLIO only goes to 22 kHz. The XT has a slightly "darker" presentation compared to the 7000 and despite the two versions being tuned to render the same frequency response +/- 0.5 dB. The rise in impedance at Fs is much higher for the 7000, thus my guess on a thinner coating.

Jesper has nerves from steel. The 8531 drivers are ~2 mm to the edge. All drivers on a vertical line determined by the 8531 drivers.
Two vents are needed, 65 Ø x 190 mm placed with center 160 mm from top and 250 mm from bottom. The ports should provide a box tuning of 32-33 Hz as can be seen from the impedance measurements below.
This provides +1.5 dB @ 50-60 Hz. If you want a flat tuning, go to Fb=22-23 Hz, that is: One port and ~18 cm length (stuff the other port).

Cabinet dimensions here doesn't include base plate.

Jesper's calculation on internal square area says 6.3 dm^2. With an internal height of ~114 cm, we have a net volume of ~72 liter. The net mid cab volume is 4 liters, lets assume 5 liter brutto, thus 67 liters for the bass drivers minus bracing, ports, crossover, etc. In total a net volume of 65-66 liters, exactly twice the volume of the smaller Ekta. Plus/minus 1-2 liter in this size cabinet doesn't matter, so don't worry.
I've had a report from an EktaG builder, who made 45-50 liter cabs and reports a too dry and prominent lower/middle-mid midrange, that is the 160-640 Hz range and I'm pretty sure this is due to the reduced volume. I once tried a single 18W driver in a 22 liter cabinet and it had a significant impact on midrange performance. Too forward and too dry - to my ears. Apparently the 18W needs room to breathe.

Now, if you have never made a cabinet before, the Ekta Grande is hardly the place to start. For the more experienced builder the above dimensions should be all needed to replicate Jesper's dream speaker.

Should you want a pair of Ekta Gs really bad, Jesper will make a pair of cabs for 1,000 €. These will be finished in sanded MDF and be ready for lacquer. So you have to engage an automobile painter to undertake the job - or DIY.
Jesper can be contacted at: spohrjes@hotmail.com

Left: D2904/7000 tweeter response on Ekta-G baffle. It doesn't get much better than this.
Right: Impedance of 7000 tweeter. The high peak indicates a softer suspension compared to XT25TG tweeter.

Left: XT25TG-30-04 tweeter response on Ekta-G baffle. Flat as the XT25 use to be.
Right: Impedance of XT25TG tweeter.

Left: 12M/4631G00 response on Ekta-G baffle. Minor bump at 800-900 Hz and minor dip at 550 Hz.
Right: Impedance of 12M/4631G00 middriver. The 12M has a remarkable high resonance peak, indicating high Qm, and so it has; 5.57 according to SS claims.

Left: Response of one 18W/8531G00 on Ekta-G baffle. Remarkably extended and a smooth decay. Very few 6½" driver can manage this. Right: Impedance of 18W/8531 driver (one) in cabinet.

SPL response of 7000 and XT25TG versions. 2.8 V @ 1 metre distance. Response is merged with bass nearfield response at 350 Hz.

7000 version with mid connected with same polarity. Response taken at between-mid-and-tweeter height a 1 metre distance.

Connecting XT25TG tweeter with negative polarity display a deep suck-out at ~3.5 kHz = point of crossover.

Impedance of complete system with 7000 tweeter. As can been seen we reach an impedance of 3 ohms from running two 18W bass drivers in parallel.

Inpedance of system with XT25 tweeter. Not much difference from 7000 version. Wasn't expected either.

SPL of drivers @ 1 meter/2.8 volts, driven from crossover, plus summed response merged with nearfield response at 350 Hz.