if hes not tall, he can slide the seat forward, enough to make it a good 5" deep. you can make the rear of the enclosure out of fiberglass. that will clear a good .75" and increase the enclosure volume. if he cant have the seat that far forward. hes going to have to go with slim subs. and thats still going to need at least 3" of clearance. he can do it, just not sure if those sub will work for it.
if you had some pics with measurement line drawn on it, it would make it easier for us.
laters