If there's enough room above the window, I would do curtain panels that could be pulled open during the day to show the arched window, but are above the window and could be closed completely at night.

{{!gwi}}

I would make a cornice out of foam insulation board or plywood in the arch shape. I'd paint it to look like a road with a double white line in the center and attach something car-related--red brake lights for example along the arch and hang curtains as daisyadair has described.