The design consists of the features of shape and configuration of the whole of the Filter Media Retainer shown in the drawings.In the Drawings:FIG. 1 is a perspective view of a filter media retainer in accordance with a first variant of the design engaged with an underdrain assembly, showing a first set of passages.FIG. 2A is top view of the filter media retainer of FIG. 1 in accordance with the first variant of the design.FIG. 2B is a bottom view of the filter media retainer of FIG. 1 in accordance with the first variant of the design.FIG. 3 is a perspective view of the filter media retainer of FIG. 1 in accordance with the first variant of the design, showing the first set of passages.FIG. 4A is a detailed, enlarged fragmentary view of a plurality of compartments of the filter media retainer of FIG. 3 in accordance with the first variant of the design.FIG. 4B is a detailed, enlarged fragmentary plan view of a plurality of compartments of the filter media retainer of FIG. 2A in accordance with the first variant of the design.FIG. 4C is a detailed, enlarged fragmentary bottom view of a plurality of compartments of the filter media retainer of FIG. 2B in accordance with the first variant of the design.FIG. 4D is a detailed, enlarged fragmentary bottom view of a plurality of compartments of the filter media retainer of FIG. 2B in accordance with the first variant of the design.FIG. 5A is a front view and rear view of the filter media retainer of FIG. 3 in accordance with the first variant of the design.FIG. 5B is a right side view and left side view of the filter media retainer of FIG. 3 in accordance with the first variant of the design.FIG. 6 is a transverse cross-sectional view of the filter media retainer of FIG. 3 in accordance with the first variant of the design.FIG. 7 is a perspective view of a filter media retainer in accordance with a second variant of the design, showing a second set of passages oriented perpendicular to the first set of passages.FIG. 8A is a top view of the filter media retainer of FIG. 7 in accordance with the second variant of the design.FIG. 8B is a bottom view of the filter media retainer of FIG. 7 in accordance with the second variant of the design.FIG. 9 is a perspective view of the filter media retainer of FIG. 7 in accordance with the second variant of the design, showing the first and second set of passages.FIG. 10A is a detailed, enlarged fragmentary view of a plurality of compartments of the filter media retainer of FIG. 9 in accordance with the second variant of the design.FIG. 10B is a detailed, enlarged fragmentary plan view of a plurality of compartments of the filter media retainer of FIG. 8A in accordance with the second variant of the design.FIG. 10C is a detailed, enlarged fragmentary bottom view of a plurality of compartments of the filter media retainer of FIG. 8B in accordance with the second variant of the design.FIG. 10D is a detailed, enlarged fragmentary bottom view of a plurality of compartments of the filter media retainer of FIG. 8B in accordance with the second variant of the design.FIG. 11A is a front view and rear view of the filter media retainer of FIG. 9 in accordance with the second variant of the design.FIG. 11B is a right side view and left side view of the filter media retainer of FIG. 9 in accordance with the second variant of the design.FIG. 12 is a transverse cross-sectional view of the filter media retainer of FIG. 9 in accordance with the second variant of the design.The broken lines of the underdrain assembly shown in FIG. 1 are provided for the purpose of showing environment only, and form no portion of the claimed design.