Make the initial investment and don't ever question it he said he wanted bomb shelter status- double walls all the way around why not invest a few extra dollars to fill the void between panels? pretty simple mix and pour if it were my box I'd do it
Did you happen to see an "storage size: 13.9cuft" in the car specs? I would bet $ that after the box is built and woofers are in your looking at half that, maybe a bit more if you use glass and get creative