Here's a little trick to hold your sub in place. I bought some strips of velcro that are probably 2.5-3 inches wide and cut pieces about 8 inches long. I then stuck together two pieces of the "hook" side of the velcro. So now you have a single 3x8 pice of velcro that is "hook" on both sides. Stick one or two of those on the bottom of your sub box (assuming it's carpeted). Then stick the box on the carpet that's already in the trunk. Works like a charm and you can pull it right out if you ever need to remove it.
2013 Mustang GT, Flow master blasters, J&R LCA's, BMR Panhard bar, Eibach pro kit, FR500 18x9, 18x10 in black chrome.