i use the trunk area all the time, i have 2 small kids and im always hauling stuff back there. No need for protection...lol. I have 2 small dings in the cone now, but ive had this setup for over a year now. All the panels are made of fiberglass and are tough and the speaker is composite/metal material. I have a knob on my dash next to my steering wheel to control the gain of it as well, i can just turn it off if i have a load back there.
No offense bro but... this sort of reminds me of my golf game. I wack the s*** out of the ball and drive it 250+ yards usually down the fairway. Then proceed to hack the ball into the woods and double bogey.
You put all that effort into building the box and I'm assuming the sub was a few bills... why not just finish the job and slap a grille on it? Leaving that poor sub out there unprotecting is just asking for trouble. I've seen some MS3s with gigantic boxes in the back. To me, that completley defeats the purpose of having a hatch! I cannot wait to load mine up with... whatever. Having a 70lb dual 12" sub box in the back obviously makes that difficult... so at least you didn't do that! But your sub is still left unprotected... so I'm not sure what's worse! lol
Anyway... do what you want! hell, it's your money. I'm just sayin