I am toying with an idea about a low profiled dual 18" ported sub.

But i am a bit in the woods about how i should sim it.

Please have a look at the pictures...

Is it a bandpass type or just straight-up ported box?

I am also thinking of ditchin´the outside port and go for a slot port that ends behind the woofers instead of infront of the woofers.

The ported chamber will have around 360 liters internal volume.  

Not sure what about it you would sim as a horn, the only significantly tapered section feeds into the rear chamber so would just act as rear chamber volume and isn’t long enough on the “outside” to provide any real horn loading effect

I would just sim it as ported if the baffle was tilted less to around 45 degress. But this is 82,5 degress so very close to EV MT2 manifold as you point out Roadrunner.

I haven't picked out a driver yet, but I recon I would be needing a very strong cone and suspension. Something the EV MT2 didn't have. 

Why would it need to be tapered to act like a horn? Its easier (and probably more truthfull to practise) to simulate it as a 2 segment ODH. You will definately see gain from it (being usable is another story). 

By just looking at you drawing and doing some gueswork I simmed the following:

And simmed as a standard BR:

While tuning is really low due to the small port area and relative long lenght, I can say that the real life response of the cab is more in line with the first sim. It acts like a large resonator (comparable to the paraflex high tuned resonator) and adds quite some gain higher up. How would you sim it as a 6th order bandpass? How would you specify that chamber?

I would say in real life its a bit between the two sims eventually, the gain from the cavity is imho not small enough to just ignore it.

Because that is fundamentally the difference between a horn and a port? Without providing any actual horn loading it is just a resonator.

Did you model the front chamber area on that BR plot?

It’s too short and the CSA too large for the drivers being mounted flat to effect it beyond the margin of error of a HR sim 

It is possible to make them wider without causing to much hazzle when deploying them at the venue, so i think i have come up with a more simple and at the same time a bit cheaper design to build.

Outside dim: 1236mm wide, 1400mm deep and 425mm high.

Baffle slanted at 35 degrees from horizontal.

Two slot ports. Each 110x189mm. Minimum depth need to be 590mm de clear the baffle.

Total internal volume around 440-445 liters when bracing, handles etc. etc. are all accounted for. Note that the depth of the enclosure isnt fixed. There is plenty of space behind where the enclosures will be positioned.

Would this design fit under a BR type design? it almost as close as it can be.

I guess two enclosures would do the trick, but if this current width is enough it is possible to fit a third enclosure. Headroom is nice to have 

As a start i will be aiming for 18sound 18LW2400 as it is cheap to replace and does a good job. Proven and reliable.

Mind you that this is only a rought idea. Fine tuning will be the next step so the enclosure will be made to fit the driver of my choice.

One thing you might consider to protect the drivers from drinks, etc. it to mount them the other way up, at that point the floor may cause a boundary effect that you need to consider in conjunction with the little chamber as the boundary will artificially increase the size of that chamber. Likewise boundary and inter-box coupling of the slot ports may be something to consider, depending on how anal you intend to get with this design.

Only used 15s for low/mid in top box enclosures. But I guess it should be possible to achieve the same result. It might be possible to fit 8 drivers in the same space as the 6 18s would occupy.

But I would be a bit hesitant as I never have heard a 15 deliver the same kind of bass as a 18. 

After you asked that, I thought a bit about it, and I actually could use some compact enclosures with small frontal area in more than one occasion, so I will have a go at a dual 15" design. Actually if I could get under 95x40cm frontal area, I then got three other places I could make some clean setups with visible speakers. 

I might go with something like B&C 15SW or DS 115. Plenty of power to drive them, so can't see a issue with big boy 15s like these. 

You would have to be very careful designing a grille to go around the edge of 15" drivers if you want total cab width to be ~40cm. The drivers alone are 39.3 so would already overlap the side panels, meaning you can't extend those forward of the baffle.

You'd likely also have an issue fitting enough vent area on that sized front panel to make full use of big boy drivers.

If you have enough depth available, you might angle the baffle to fix #1, though that adds to the complexity of the woodwork a bit. (Edit - just remembered you were already contemplating that, so perhaps not an issue for you.)

But your right about port area. That's not gonna do well with the limit amount of front area. 

But first of all, the goal is to get low and loud. Size doesn't matter that much in the end. As long as it can fit under the stage-ish location they will be deployed at.