As long as you leave enough space between your plants and the box, the height isn't that big of a deal. Camellia plants are a good choice in a shady situation. You could even start with taller ones to help hide it better. You could use about 3 of them there and then in front, to fill in that whole square area, a few semi dwarf azaleas so you get 6 months worth of flowers each year.

If you wanted something a little taller and something that needs no special care, is insect free, and lasts forever, ( can literally live to be 300 years old) I'd put in a few Ceratozamia hildaes. (The Bamboo Cycad) They even laugh at 12F, which you may never see.