Hi,
either you use a 2d ui framework such as 2d tookit, or Ngui to have letters shown as planes that you place around a cube.
Or you have your owne set of textures for each letters, and you also have your cube with childs representing these letters.
I suggest you go with a 2d ui framework, cause it will atlas fonts for you and let you enter the character as a text, it will be easier to change the content etc etc.
bye,
Jean