The trouble with perennials in planters is the freeze and thaw cycle which will kill plants. Making them big and lining them with styrofoam helps. On the south side there will be a lot of freeze and thaw, because the sun beats down in the day and then it freezes at night.

Clematis likes to have it's head in the sun and it's feet--or roots--shaded. If you want the look of the boxes, make them without bottoms and plant the Clematis in the ground and put some mulch around it. The box will shade the roots. The mulch will keep weeds out and you avoid the danger of freeze and thaw.