X: A creative solution. Does the rear door resonate too much? I filled mine with structural foam.
I won't say that anyones sub is wrong without listening to it first.
My experience is that setup in a properly designed box a single 10" is enough.
For the space available I think you will find a single 10" will sound better in the larger box rather than squeezing in 2 drivers.
It depends on what you expect it to sound like too. I want deep fast bass, and will not tolerate 'boom' Not to say my system can't play loud, but its SQ not SPL.
I chose to give up the space behind the rear seat for my sub box, and retain the spare tire. I'm contemplating removing the rear seat and making a MDF storage/sub/amp rack.
John C.