A downhole tool, such as a fracture plug used during a fracture operation, installs in a downhole tubular, such as casing. The tool has a mandrel with a sealing element disposed thereon between uphole and downhole ends. Slip assemblies on the mandrel can be moved to engage the downhole tubular. The slip assemblies have inserts. The inserts have a body defining an internal cavity at least partially therein. The body is composed of a ceramic material having a first density of about 3 g/cc to about 6 g/cc. A volume of the insert can be about 0.4 cc to about 0.6 cc, and a mass of the insert can be about 1.2 g to about 3.6 g. In one example, the ceramic material is silicon nitride. The cavity can be filled with a low density filler material.