I’m thinking this might the go. I can fold down the centre section when it’s just me in the car 99% of the time. I can also remove it if we need extra space for going in holiday.
https://audiotechdirect.com/fit-box-...for-bmw-5-g30/
I wouldn’t cut out the section behind the cup holder drop down section. Just fold the whole centre down. It looks like it will stay in place pretty well.
And this subwoofer would be a good fit in the box and would work fine with the ACM amp.
https://rockfordfosgate.com/products/details/p2d4-10/