Nice build. That sub box isnt moving unless you accelerate very quickly. I wouldn't worry about it touching your taillgate trim at all. If it does, itll be with very little force, not enough to damage the actual sub at all. The port may be a little small. You should consider either making it larger, or sealing it all together. Obviously, a saw will make it larger. Or, you can plug it with a large shoprag, soaked in silicone. A smaller port can often make whistling sound as large amounts of air push through a small amount of space (like when a person whistles through their mouth!). A larger port will make a louder, less accurate bandbass sound while no port will make a smaller, but much tighter and more accurate bass sound that will sound nice and clean