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Joe X design is a good ported box. Corner 45s don't do much for a subwoofer box besides brace it and eat up internal volume. If you wanted a similar design with a bit more internal volume then I do have one free on youtube. Feel free to do basically whatever with it. You could also try to design a mass loaded transmission line with offset driver/stub. It's basically a lot like a ported box but attempts to cancel resonance in the enclosure to give more bandwidth.

Here is a box made to be similar to the one you linked but a bit better for the lows and the cutsheet can be completed just with a table saw and a jigsaw, no need for router: Power: to sub(s) rated, amp subsonic filter to 28Hz, minimum port clearance 4", figure is just an assembly guide:

I would definitely look into seat lift kits to fit a box of proper size for those subs, make that enclosure with 3/4" birch wood, dual layer for the baffle, proper bracing inside, specs are 4 cubic feet net 64 square inches of port area tuned to 32Hz, I believe the sub cutouts need to be 7.34". Depending on how much space you find to work with I could fine tune that spec to increase output and low end extension as possible.

That box is designed for SA-8s as a result the X subs are going to find themselves in a too small box with low port area and tuned too high, surely above the 36Hz specified as driver displacement for X subs is larger, maybe it will get to 38Hz or more and could experience some port noise. Used to a large box tuned very low you will definitely notice a loss in bottom octave bass. Regarding to your question, you can always strengthen the box but if it is made of cheap thin 5/8" wood then it is not impossible. Lastly the pre cut sub holes cannot fit X subs, you will have to work on the holes. Overall that box seem expensive and not adequate for the subs. I would lift up the seats and do a proper custom enclosure for those subs and even then you probably have better bottom octave bass right now than you will ever get with some 8s.

Most component set passive crossovers don’t include a high pass for the midwoofer. I would check to make sure but it’s likely you need to high pass the amp’s output.

Sorry I've been MIA. As for the 74AH pro I'm not sure you should get it. I'm second guessing my purchase already and I cant send it back. I'm still going to try and use it but i wish I would have done more research. Apparently these cells have a really narrow voltage range. It might work for me but I'm still a ways off from testing them out. ----------------UPDATE 2/25/24--------- I decided to revamp my system entirely. I sold everything from my old car except 2 SA x-8's, my SIA2500.1d, and my wiring. I got rid of my Infinity Kappa 3-ways and purchased Infinity Kappa Perfect 3-ways. The new setup is as follows Helix M Four DSP running my tweets and Midrange 2 Sound Digital 800.4's running my midbass, center channel, and rear speakers SIA2500.1d running 2 SA X-8v.4's - 2ohm's @ 2000watts - Also contemplating building a T-line enclosure for these Also I decided to do custom a-pillars. So on with that! These are my Infinity Kappa 300m midranges all taped up and ready to be trimmed. The two things i dislike about these are that the mounting tabs add an inch of width and they don't come with grills. Gonna have to fix that. Left one trimmed, on the right you can see the problem. These are some funky a pillar pods I found on amazon. I'll still need to do some fiberglassing and trimming once my second set of a-pillars come in. Not gonna modify the originals. Thank you skar audio for the sacrificial grill Fit's but is not done. This is a tinner's hammer hammer. Time to shape some metal Not perfect but you can see what I'm going for. A can of zyn fit's nicely to help with the shaping Also had some steel round mesh from another project So I decided to try that as well Side - by - side of the two grill options Now for the tweeters. The flush mounts we too big so originally I was going to use these. But it looks hideous and bug-eyed. So I trimmed one of the flush-mounts to fit. And ditched the black grill. The steel looks better imo, especially when matching the tweeter. There's still a lot I need to do to these. Currently the grill is sitting on the midrange surround so I need to fix that, and the pods still need to be fiber glassed to the a-pillars and covered with some kid of material. But that is for another day.

Finally got some screws so I could throw the subs in the box and see how it fits in my car. First issue I noticed is that the terminals + wire was too difficult to get into the box without ripping the wires out. So a couple quick relief cuts were needed. Then I hit it with the shop vac. After that, I wired in the subs and chucked em in the box. I pre-drilled some pilot holes and screwed them in. They're snug but not fully tightened yet. I have an idea to make the box a little prettier so they'll probably come out at least once more. And lastly just a quick test in the car. I'm gonna drive around with them for a few days to see how the space effects me. This is the average load I have in the trunk with my tool backpack, a set of rain gear, and the Milwaukee bag full of drills. Sometimes I carpool so you can just about double that for those days. I'll need to figure out something to protect the subs. Finding grills would be difficult with how much xmax these guys have. Maybe some bars or something. Shoot me any products or ideas you guys have. I'm also thinking about adding some kind of small lip to the top of the box so I could throw My toolbags on top of the box and not worry about them sliding off. Maybe some kind of cargo net. who knows. And DEFINITELY glad I'm going with 2 subs now. This box loaded is a back breaker. I May throw some straps on top to make it easier to pull in and out. 3 subs would almost require two people.