Need help with enclosure ideas for 2 Skar SDR18D2's in the trunk of Lexus LS430

[UCF30VIP]

I recently sold my two Skar SDR12D2's in their Harmony Audio ported enclosure w/ 5.5 cubic feet & tuned at 30hz that I was pushing with a Kenwood KAC-9106D that I had wired at 1000watts RMS / 2000watt MAX @ 2ohms, the subs are rated at 600w RMS/1200 MAX so this should be just under their limit if the amp puts out true power, it seemed to push them hard with the gain turned at 3/4, all bass boosts off, HPF @100hz and subsonic @20hz, I originally was running an Audio Dynamics LC2i LOC to power the RCA's from the factory Mark Levinson subwoofer amp to my Kenwood amp but recently installed an aftermarket headunit that has a dedicated subwoofer preout with subwoofer control on the headunit so have been switching back and forth between the headunit and LOC to see if one sounds better than the other but havent been able to tell really. 

I picked up two brand new Skar SDR18D2's which as far as I can tell are exactly the same motor, 2.5" voice coil, and power handling as the SDR12's that I was running but obviously a much larger cone area. I am unsure of what kind of box I should run these 2 subs in, they are going into the trunk of a Lexus LS430 that is sealed off from the cabin except for the factory subwoofer location where I removed the 8" subwoofer in an attempt to port some of the sound into the cabin. I have a fairly large trunk and should be able to run up to a 10 cu. ft. box if needed. Skar recommends a 2.57 cu. ft. sealed box or a 4.6 cu. ft. ported box tuned at 32hz.... ive played around with WiniSD and SpeakerBoxLite.com trying to come up with the best box design and am hoping somebody can chime in and help me out with some specs, or info that might help. 

Ive attached some pics of the trunk and just some cardboard visual aids that kind of show a couple of different mounting options but are no way my limiting factors, I am open to anything, I want the loudest bass possible from these, I want them to at least be an upgrade from the SDR12D2's I had in a prefab box lol. I have several sheets of 3/4 x 24" x 48" MDF and 1/2" x 24" x 48" MDF for bracing, corners, port etc.... 

Any info is appreciated! 

[Joe X]

Probably you can do two sealed or one ported but not two due to all displacements, get a copy of Torres calculator to figure out your net internal volume of a particular design.

 

 

Link to Torres calculator: https://bit.ly/Tvj3gB

[UCF30VIP]

Well I ended up trying a 5.7^3 ft sealed enclosure for the 2 Skar SDR18's, it sounded ok but wasnt near as loud as my 2 SDR12's in a 5.5^3 ft box. I went ahead and cut a port into the box and vented that into the cabin w/ a 10" Diameter port running through the factory subwoofer location into the cabin and it still wasnt near as loud. I pulled one of the 18's out of the box and finally it was just as loud if not louder than my 12's were. I wasnt happy though so I sold both the Skar 18's and the box and bought a Skar EVL18 which could handle 2500 watts, has a larger voice coil, much larger motor and double the xmax @ 23.5mm one direction. I figured if I only had enough room for one subwoofer I might as well make a correct box for a better quality subwoofer. 
I ended up utilizing my spare tire well since I wouldnt have access to a spare tire with a box this large anyways, and I was able to build some of the enclosure underneath the rear deck so I could run a port straight up into the cabin. My first box design was a 4th order, I honestly dont remember the specs of it right now but I wasnt impressed and tore it down and restarted again building just one large 12.3^3 ft enclosure that consumed my whole trunk. I then upgraded my amp and wiring from the Kenwood 2000w monoblock amp to a Taramps 3000 watt amp, this setup has much more bass than anything I have previously had in this car but nowhere near what it should have. 
Is the issue simply not being able to get enough of the sound into the cabin? I only have a single 8" diameter hole in the rear deck to allow the port through, would it be better to port into the trunk and just let everything from the trunk come through that 8" subwoofer location or should I continue to port through it as I am? 
I just got done with a friend that built a single box for his Skar EVL12 in a GMC suv and it is MUCH MUCH louder than my EVL18 setup. I ordered another EVL18, bought an air nibbler and am ready to cut the rear deck through, I dont use the rear air purifier that often, and I dont use the rear AC ever, unless I have something in the rear fridge so I am going to remove the rear AC and air purifier, cut the fridge open so it has access to the trunk and allow as much sound through as possible. 
Here are some misc pics to cover some of what I mentioned. 

[mhuck123]

Any update on this or new photos? I have the same car and am working on an enclosure but not a lot of info out there