I made this large planter box (32”x28”x24”) for a 6’ Juniper tree that includes a privacy fence attached to the back. I used 1×6 PT wood for entire construction and some 2×4 PT for framing, ripping the 1×6’s down when needed for privacy fence and the trim around the box. I used my ‘kreg jig’ pocket holes for constructing box. I left bottom of box open and lined with chicken wire and landscape fabric to allow for draining and lined box with plastic to protect it from dirt and mulch.
That planter should be great for your Tree,looks great!