custom sub box and questions on placement.

i have 2 12w6 and i would like to make fiberglass boxes. my idea is to put 1 sub in each corner of the trunk(towards the rear of the car, not up against the backseat). at first i was going to make them face the cabin, but i read that this is not the best idea because of how sound waves travel and that facing the rear of the car allows for best performance.

so my main question is if i build the box inverted with the magnet free air, is that good enough to reflect the sound waves and will this sound good? also, i can fold down both of my back seats (08 mustang) so could i use this to make the subs sound better?

ive run through many designs and i want to be 100% sure that i am selecting the best one so i dont waste money on fiberglass materials. thanks for all the help!