If you assume the idiot* case, it will be fine.
(32-1.5)*(32-1.5)*2(18-1.5) = 15,349 cubic inches /1728 = 8.8cuft - driver - port
From Webstereos Dictionary
*idiot
[id-ee-uh t]
noun:
1. Informal. an utterly foolish or senseless person:
If you think you can wear that outfit to a job interview and get hired, you're an idiot!
2.Psychology. (no longer in technical use; considered offensive) a person of the lowest order in a former and discarded classification of mental retardation, having a mental age of less than three years old and an intelligence quotient under 25.
3. Average member of SMD; outspoken, but clueless. Typical behavior shown by cutting every corner possible to fit the next size up sub in their car, but calculates and fails miserably. Post install behavior is usually sad remorse followed by relentless bragging on car audio forums on how great their garbage is.
...
I will assume you are not that guy and while the moron above will only use under 2cuft, you should use much more.