I would suggest 1 15, 2 12's or 2 10's. Theres really not enough room to let the 15's breath properly. As far as subs for the price of 1 jl 13w7 you could get a pr of xl 10s or 12s that will own anything jl makes. Plz dont listen to your friend trying to steer you away from custom boxes. All the loud vehicle have custom built boxes.
If you decide to go with 10's I have several box designs that were built for your trunk.