To extend only the bottom would be problematic: the shape has to come out of the mould, and the upper and lower parts need to mate. I suppose the top shell could form the frame of the top of the box.
If the bottom half had a box molded into it, and a separate lid piece, the two halves could still mate as long as the center seam is not affected, no?
Pardon the crudity of this sketch - just trying to visualize it.