A side skirt assembly for a trailer of a tractor-trailer includes a skirt panel and a plurality of skirt mounts that couple the skirt panel to the frame of the trailer. The skirt panel can be formed by a layered composite formed of alternating layers of fiberglass and layers containing recycled fiberglass. The position of the skirt mount can be laterally adjusted along the trailer frame whereby the skirt panel can be mounted to the trailer in more than one lateral position. Additionally, the skirt panel can include a track extending along at least a portion of the skirt panel length, and the track can slide relative to the skirt mount to adjust the longitudinal position of the skirt panel on the trailer during assembly.