Thanks for uploading text and pics on specific forum page. You may go for one

of the 3 subwoofers listed below:-

As per tech specs of Sony amp online, it is bridgable... as in, you can connect 1 or 2 subwoofers, but it hasn't got much power. So in my opinion, just go for a single magnet subwoofer. Please note, all 3 subs listed above have same magnet size, 120mm outer dia. And will give good audio effect. No need for extra wiring as amp is already seated in your car. Maybe audio signal wires between amp and sub, which is nominal.

You may have to go for box fitting or boot fitting, depending upon your car design, which a car mechanic will advise you better.