So... we have these nice wooden baby/dog gates that we can put up to confine Biggles. Zoe is still too small and can just fit through the bars. For her, we were using a wire crate. However, the wire crate was too big -- she would still potty in it. So we halved the space (making it a triangle... sort of) and that works.
However...
Well, take a look: Zoe the Escape Artist
Obviously, we'll need a different solution.