Does your seat fold down? if so fire the 15's forward and seal them off from the trunk and cover your speaker shelf with a deadner. basically make a mini wall, you will have no trunk rattle and some serious bass.
If not you should just remove the 8" , even remove your 6x9s if you do not need them. If you stick a port in there i would think it would be making a bandpass doing so ,but i would just take out the speakers tho for flow into the car.