Even easier...
http://www.amazon.com/MBQUART-ONX254SQ-10-Inch-Square-Subwoofer/dp/B00852X2YG/ref=sr_1_3?s=electronics&ie=UTF8&qid=1393246680&sr=1-3&keywords=10+inch+square+woofer
You'd have to make sure it fit. Might even have a chance to fit without having to alter the box.
http://www.amazon.com/MBQUART-ONX254SQ-10-Inch-Square-Subwoofer/dp/B00852X2YG/ref=sr_1_3?s=electronics&ie=UTF8&qid=1393246680&sr=1-3&keywords=10+inch+square+woofer
You'd have to make sure it fit. Might even have a chance to fit without having to alter the box.