I suggest faking it with something akin to forced perspective. Sometimes, when you can't get something to look right, you have no choice.
Make a giant arch model and move it into place over the city. Don't make it as a circle, just a parabolic arch. Then scale it while looking from the camera's POV. I don't know which modeller you're using, so this might be difficult. Lightwave can't do this easily, but Truespace can. The idea is to keep the local ground flat, but this arch--whose ends cannot be seen from the camera--looks like a proper ringworld Arch due to the forced perspective of the local buildings.