A luminaire includes multiple light-emitting elements (LEEs); a base supporting the LEEs; and a first wall and a second wall each extending along a first direction from a respective first end facing the LEEs to a respective second end. The first and second walls have light-reflective surfaces facing each other. In one or more cross-sectional planes parallel to the first direction, the light-reflective surfaces of the first and second walls have first portions that curve in opposite directions, second portions that are parallel, and third portions that curve in like directions. The first portions are arranged facing the LEEs to provide an input aperture that receives light from the LEEs. The third portions are arranged to provide an exit aperture that outputs output light into an ambient environment. The first and second walls are configured to propagate light from the input aperture to the exit aperture.