Only option would be to get an underseat sub fitted, but it would have to be removeable so I can take it out when the car goes back in 30 months time. I already know I have no intention of keeping it over the three years.
Why not get a not so expensive sub, build a box and put it in the boot behind the rear seat ot to one sideor remove the existing one and put in a box in the boot?
From memory when we last spoke I think the Canton is a 8ohm DVC speaker?
You can buy the plugs cheap enough so your not splicing manufacturers harness and even use 2 4ohm in series.
Have you removed the USB C sockets to have a look.
However 30 months will soon fly by mate 🙂