Mhiko,

Well, if I were doing it I would not use a Geoboard, which does not seem to me to be an ideal tool for this task! An actual cube with faces divided into 1x1 squares seems more appropriate; it could be made in cardboard or heavy paper and then flattened and stuck into a scribbler. But if you must use a Geoboard,, I can see two options.

1. Use an isometric Geoboard to make a picture of the solid, and use extra bands to divide each of the three faces you can see into 1x1 squares (which won't be square on two faces). Recognize that there are three more faces, of the same dimensions, that are hidden in this pictures.
   
   
2. Use a square Geoboard to model the shape you would get by flattening the surface of the cube or prism onto the plane.

Good Hunting!
Robert Dawson