the box they show is 4cubes optimal
port is 3.5W x 30L 16H
lol, ignore the 90deg cuts if u cant do that, just make them straight, like the pic above
The outer pieces are just bracing, which u can just put a 2x2 piece of wood to do.
there angled so u dont have standing waves, but i doubt ul notice the diff, if you really think its a problem glue some insulation to the corners, or cut some pvc to round them off
i've edited this like 10 times now lol