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The problem is not the size of the cutout, but the fact that you need 1 cube per speaker of air space. Most boxes that you would build under the seat only have .75 cubes of air space. Based on my calculation, you could possibly build a box out of 5/8" MDF that would have 1+ cubes of air space per speaker - but there would be no room at all for mounting amplifiers or anything else (these would have to go under the seats.)
This is my diagram I made for installing 2 x 10" subs with .75 requirements along with 2 amplifiers:
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