A retainer ring for a light fixture includes a cylindrical main body section that defines a cylindrical axis, and an inner retaining flange that extends radially inward from a distal end of the main body section. The inner retaining flange extends substantially about a circumference of the distal end. One or more coupling features extend radially inward from the retainer ring. A customization kit for a light engine that defines an optical axis includes a retainer ring configured to couple with the light engine, and two or more primary optics that are mechanically compatible with the light engine, and retainable to the light engine with the retainer ring. When the retainer ring couples with the light engine, a cylindrical axis of the retainer ring coincides with the optical axis.