Rereading your OP. If you had an image of the long side with the grooves in it, you could use that as a texture on the one face or a simple extruded box. Then mitering it would be trivial with either a drawn diagonal and Push/Pull or just moving an edge. If you don't need the internal details at all, don't model it as a hollow shape.
[image: V15w_miter.gif]
