A culture dish assembly includes a polygonal culture dish with a bottom wall and a side wall enclosure formed by a plurality of generally planar panels joined consecutively to one another by non-planar corners. The side wall enclosure includes a generally planar top edge. Inner surface regions of a plurality of the planar panels are formed with a plurality of projections or recesses that are spaced from one another and spaced from the corners. A splash guard includes engagement flanges disposed for nesting with inner peripheral surface regions of the panels of the side wall enclosure. The engagement flanges include projections or recesses disposed for releasably engaging the projections or recesses on the side wall enclosure to hold the splash guard on the culture dish without creating stress concentrations in the corners.