@jkristoph said:
Can I push "hollow out the planter" in relation to those angles?
I think there's an easier way. Rectangle the bottom. oFFset in the width of the sides. PushPull up a hollow, rectangular planter. Double-click to Select the top face and edges. Use the Scale tool on a corner, Ctrl held down to scale about center, and pull the top out a bit.
If your modeling seems like work you're working too hard.