dude, another suggestion would be to buy some chicken wire, and then to experiment with different shapes of it.
basically, the idea i had, was to weave the plants in and out of the mesh, as they grow.
so, all the plants will be restricted to the mesh., height aswell as width.
sorry mate, but im crap at drawings, so u'll have to use ur imagination.
an arch shape should be cool, but as long as u can weave, any shape is possible.