Well that's a great idea. But you do have to remember that boxes get colder during the winter than having them planted in the grown. So I would try for perennials that will take colder temperatures than whatever
zone you are in.
Ok I'll try designing one or two
Lets see at the sides of the box to frame the window I think I would try some of the monks hood. One of them in the 2 foot range for height. This puts me in the blue/purple color group. Aconitum fischeri might be a good one. It
flowers from late summer to early fall so next I want something that will come in much earlier. I think near the front of the box I would do some of the smaller semi-evergreen ferns, then some lilies of the nile in the same blue purple range. More towards the center of the box and then fill in with one of the groundcovers such as Kenilworth Ivy. The yellow throats on this flowering ground cover will be just the hint of contrast to make it look good I think. But then I'm no decorator so we definitely need a second opinion.
Now for something in the pinks lol. Starting with the ornamental strawberries for a ground cover, with Anemone, Parsley Leaf near the sides again for the purpose of framing the window. and near the center I'd try for some of the double columbine. They are so dainty and pretty.
Ok that's about it for me being a decorator lol.