Cut the slats to 12 3/4" and chamfer the outside edges. Cut the four bottom corner blocks and attach them to the bottom horizontal pieces with biscuits so they're flush to the top of the bottom horizontals.
Dry fit the planter, minus the slats, then clamp (don't glue). Mark a centre line on the 1 5/8"-wide crosspieces. Hold each crosspiece on the outside of the planter, aligning the centre line with the inside corners of the posts and horizontals; scribe (mark) the angle, and cut to size.
Before gluing, give all pieces a thorough sanding with 120-grit sandpaper. Use an exterior glue on the biscuit joints and all the surfaces that meet, except the slats, which need to be able to expand with humidity changes.
Fit the crosspieces in place so one overlaps the other, scribe lines where the outer piece covers the inner one, remove and cut away the overlap. You will now have three pieces that form the “X”; glue and nail into the posts and horizontals.
The slats of random widths sit on top of the horizontal and corner pieces; they are spaced about 1/8" apart to allow for drainage. The outer slats get a 1" x 1" notch so they fit around the posts.
The finials truly do finish the piece. I used 2 1/2" wooden balls available at craft supply stores. They have a flat bottom complete with a 3/16" hole. Mark the centre of the top of each cedar post, then drill and attach the ball with a dowel and exterior glue.
If your planter is going to be shifted around a lot, drill a larger hole in the ball and post, and fit it with the corresponding dowel for added strength.
Fill any knotholes or imperfections with exterior wood filler; allow to dry, then prime the planter. A light sanding with 220-grit sandpaper will smooth the raised grain created by the primer; remove any dust with a tack cloth and vacuum, then apply two coats of an exterior acrylic paint.
Now your plant has a home fit for a king. Just don't show it to your boss.