How deep is it from top to bottom?

ferns or other shade loving plants are a nice idea.

The depth us approx. 2 feet

The no width rules out Fatsia then - Fatshedera lizei meets the criteria, if you provide some sticks for bit of support, that gets 4 feet, does well in shade and there are variegated versions available. Box - if you want to spend some money, a toparied one, maybe a spiral or similar. Many other plants that would be okay in shade can't be used because they get wide as well as tall.

There is an ilex with tiny leaves and a very narrow columnar habit that would do well in your container. While it is young you can brighten up the area round it with annual begonias, and perhaps pop in a few miniature daffodils for Spring. There's also a fastigiate berberis with golden leaves which would look nice - its not as narrow as the ilex but its deciduous while the ilex is evergreen.

The berberis won't colour up though, Steragram, not in shade (if you're thinking of Red Pillar or Helmond Pillar), they need sun, so they'll just be plain green.

Bamboo I meant the one with golden leaves not the red ones - I have both so I'm not confusing them.

but don't the yellow ones stay green in shade as well? I've never grown one, so don't know...

Well mine gets sun until mid afternoon and so I can't really be sure. The leaves round the back of it which will be in shade all the time are still golden though.

Ooh, that's interesting, thanks for that Stera.