[Mystake]

im thinking of just making two removal braces made outta wood.

can't really think of the words to describe it, but it's basically a piece of wood with an angled V in one end, and an L on the other, so that it would fit into the corner of the trunk, while cupping the corner of the sub box. Put one on each side, towards the rear of the car and it should stop it from sliding side to side and back and forth.