Remember, it's 18 feet tall.
Use plenty of wood. Metal is expensive and requires skill to work with, but it'll likely end up looking nicer.
My guess is you should probably only make the exposed portion, and build it so it's at an angle, sticking slightly backwards, as well as up. This will eliminate the useless portion that is underground, and make it look like it fell 100 miles and landed in someone's house/garage/backyard/et cetera.