I had the same issue. I messed with flipping the spare the other way around. I dont remember which direction it ended up as but one way was better than the other. Also, I put three rubber spacers to lift the sub off the tire a bit to allow room for the cable.
I think i also ended up tucking the sub under that plastic lip. The floor of the trunk rocks a little tiny bit but its good enough even for my OCD.
Thanks for your reply, much appreciated.
I'll get the bits on order and give it a go then.
Now just the dongle to sort........