Leaderboard

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.