The C pillar bar looks like a good idea. I'd want to find a way to triangulate it down to the trunk floor, if not an "X" at least a bar at the C pillar, one across the trunk floor and another going diagonal between the 2.
I'm sure a C pillar bar from another car would fit, just going to take some trial and error and measuring until someone comes out with an actual setup for a 2/Fiesta.