This install is looking retro-awesome already (being a guy from the 90's).
Waveguides and at least some discussion of large-cone IB subs, I feel like you are just one giant turbocharger short of Richard Clark's famously undefeated Grand National SQ champion car!
Since you can't go IB or Aperiodic (his was technically AP), and since you already own some pretty awesome monster cone subs, I feel like I might have a great suggestion here. Or maybe two:
Option A) Clamshell isobaric.
Kicker's Solobaric subs really were the industry game-changers. Before that, boxes needed to be "Oh my god, how big?" to perform well. After the Solobaric high-mass, low compliance, high-motor-strength recipe hit, everyone started making "small box subs". Today it's hard to find anything but that.
I say that, because when I started, just about every other install I did was isobaric, purely to get the box size down. That need really went away after "small box subs", so isobaric installs all but disappeared.
Bear in mind it's not just space savings - there's SQ benefits here as well, at least if you clamshell them face-to-face (the easiest way). Because in that orientation, you always have one sub moving forward, and one sub moving backwards, you completely iron out any non-linearities of the suspension (half-roll surround moves differently "in" vs "out", cup spider can also show differences), essentially creating a perfect suspension.
And the thought of two 15's in a box in a Scion might be laughable - I'm guessing 6 cubic feet (3 each), even sealed? (I did no modeling! Pure guess)
Well, if you do a clamshell isobaric box, that turns into 1.5 cu.ft and you'll get exactly the same response curve. It's half the enclosure size of one sub - or 1/4 the enclosure size that the two subs would need.
The ONLY downside that I can think of is that you only get the SPL of a single cone, because you've essentially coupled both motors and suspensions to one cone, so you have one super-controlled single cone stimulating the air...
But is that really a downside? You've got two 15's - so it'll be as loud as a single 15 in a Scion hatch, and that's still potentially huge output - and it'll sound better than these subs even can sound on their own.
The old argument against isobaric used to also be "you have to buy two subs", "you have to power two subs"...
...but in this case, you already own two subs, and their power. This is the "free" option.
Option B) Passive Radiator
Again, I'm thinking of your available space - in this case, this would be the "more SPL/lower SQ" option, but still should be pretty good for SQ if you can dial in the PR's.
Run a single 15" sub, but this time with passive radiators to make the box tiny.
Specifically, I'm thinking of this: https://www.amazon.com/Earthquake-So.../dp/B06WWDYV7T
I don't own that specifically, but I do own those passive radiators. They are impressive in their excursion capability, easy to add/remove mass for tuning via a bolt, and this arrangement is super-clever. It ends up looking like a slot vent, hiding the PRs inside the box so you don't need so much surface area.
In this case you would sell one of your subs... you'd have to buy this, and you'll have to do a little modeling (I have the spec sheet for the PR's, I'd be glad to share), but passive radiator boxes can also be tiny - I'm thinking also in the 1.5 cu.ft. realm (again, I haven't done any modeling, just educated guessing). They don't make a 15" PR, but the dual-12 PR should be enough surface area to allow enough PR excursion to match the displacement of a huge 15.
It's the "higher SQ" version of a vented box, if you will, since there's no port noise.
Option C) Both
Like always, there's always options to go crazy, to push more purely into the SQL realm (isobaric sealed box), to push more into the SPL realm (isobaric vented box), or somewhere in between (passive radiator), even a super tiny option (isobaric passive radiator combo).
Don't be afraid to model isobaric - most software allows you to do it. WinISD does, including "isobaric" and "PR" at the same time.
If you use something that doesn't - just edit/save a second copy of your subwoofer driver file with the VAS cut in half. Now you are modeling an isobaric pair, rather than the single version of that subwoofer.
Pretty easy.
Can't wait to see how this turns out... wish I could hear it, I've never so much as seen waveguides before, though I own some horns for home use. Was always curious how they compare.
If you are anywhere near Buffalo NY, I'd be glad to help.