A fence slat is provided that can be inserted in the channels of a wire mesh fence. The body of the slat defines, in a cross-section cut perpendicular to the long axis of the slat, a substantially hourglass shape having a central narrowed portion between a pair of wider portions. The slat body includes front and back walls and side edges with an internal cavity within. The front and back walls have concave outer surfaces and the edges may be rounded or squared. One or more ribs within the internal cavity of the slat body interconnect the front wall and the back wall. The hourglass shape provides a narrower central portion for the slat body as compared to the width at the edges and the ribs, when positioned in or nearer the central portion are narrower than the width at the edges. The wider edges provide sufficient width to be held frictionally within the wire mesh channel, while the narrower ribs require less material than would be necessary for a uniform width.