Plaster-of-paris (POP) "jackets" are used by vertebrate paleontologists to protect fossil specimens from damage during transportation to the lab. Because large specimens can weigh over a ton, it is advantageous to reduce both the weight and cost of the jacket by adding fillers as in the plastics industry. Using equations published by 3M, an Excel spreadsheet has been designed that allows the instantaneous calculation of the cost and density of various fillers mixed with plaster of paris in varying ratios. Theoretical evaluations have been made for hollow glass microspheres, expanded perlite, diatomite, calcium carbonate, glass beads, glass fiber, Fiberglas (R-11), polyester resin, and epoxy resin. The key to the design of a lightweight jacket is the Cost/Density Ratio because it costs more to develop and produce a low-density material; however, impressive weight and cost reductions can be achieved nonetheless. Only R-11 Fiberglas, expanded perlite, and microspheres have Cost/Density Ratios greater than 1 and are, therefore, candidates for applied research on the design of lightweight jackets. Respective density and cost reductions are as follows for a POP/Filler Mix ratio of 5.5: Fiberglas, density reduction of 83% and cost reduction of 28%; perlite, density reduction of 82% and cost reduction of 31%; and hollow glass microspheres, density reduction of 58% and cost reduction of 26%. Recommendations include simple instructions for determining bulk densities and actual costs of formulations prepared under laboratory conditions.