Finally finished the box for now! Painted gray, all 45 degree angles inside the box, and it is now in my car! Probably going to try and mount the amp soon but not for a little bit, I want to wait till I have the power wire for it. But for now, here it is:
The box is two layers of 3/4 inch particle board on all sides except the top which is three layers thick. All sides also have threaded rod pulling them together.