You speak of art, portability, and cost as concerns. Now, I realize that this will not help with your desire for alternative art, but if cost and portability are major concerns for you, there is an option that cannot be beat. Choose anything of a reasonable size (I have used all sorts of objects; it really doesn't matter. You could even use unused mage wars markers) and place one small object at each intersection of the board, like so:
+ + + +
+ + + +
+ + + +
(The objects are placed on the + signs) Then, it is easy to picture the zone border lines between the intersections you have marked. As a huge added bonus, you can easily increase the size of the zones by simply moving the intersection markers. You can also easily make different board configurations.
Again, I realize I am not directly answering your stated question, but I think you should consider options aside from board art. Players really don't spend that much time looking at the board (at the board itself, I mean; they care more about the cards on the board), and you will find that you do not miss it at all when it is gone.