First, we need some info on the subs and the amps, as in brand names and model numbers.
As far as a box........building one is by far the best option. You'll be hard-pressed to find a pre-fab box that simply houses two 18's, much less one that works to the specs the subs and user demand.
And if the amp is pushing 2000RMS....................I think it's safe to say he'll need a new alternator. If he insists he won't need one, tell him that you won't pick him up and give him a ride when it dies in 2 days.
"The world is full of people who want nothing short of perfect,
but settle for less, blinded by their quest for purpose."