A light fixture assembly for mounting on a mounting surface of a building material includes: a light fixture. The light fixture comprises a main body comprising a back side surface and a light transmissive side surface opposing the back side surface. The main body has a first dimension from a first lateral edge to a second lateral edge opposing the first lateral edge that is greater than a second dimension from the back side surface to the light transmissive side surface. The assembly further includes a mounting adapter provided at the back side surface, and a plate spring provided at the back side surface and extending along a first direction from the first lateral edge to the second lateral edge. The plate spring is configured to apply a restoring force to the first lateral edge and the second lateral edge.