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It's not hard to DIY your own box out of MDF, especially if you make it square. The footprint of the bottom piece is below, for a stowed hatch to clear the box it has to be under 9" tall at the point that's 9" behind the hump behind your seat.
Do the math first for the sub volume requirements and amp footprint, then buy the gear once you're sure the box will work. Hardest part is carpeting it if you get the good stuff, ordinary felt is simple.
Another one of mine along the way. The sub had to be angled so I could still get the hatch stowed.