Originally Posted by kaput

Not true. I think you could get away without drilling anything honestly. I think a little dampening of the lid might be in order, but it can't be that hard. Maybe a sheet of dynamat placed above where the woofers are, or in strategic places. Also, it might be possible to raise the floor 1/8 of an inch with rubber supports to let the air flow and cut back on vibrations, while still having the floor operational.

If it were me I would either do that, or attempt to create a custom box under the front seats. The room is there, but I don't know how the seat moves and how much room there actually is to add to it. It can't be as tight of a space as the sedans. Some woofers don't need a lot of space. Maybe look at a free air sub, or something that can take .33 cubic feet of space and play loud, like this:

https://www.amazon.com/Pioneer-TS-SW.../dp/B00JQTU434

I can see 2 of these being installed under the front seats, with a bit of custom box work, you could lose no room and get great bass. Just a thought.