A simple model was devised to estimate the number and sizes of crevices visible at the surface of submerged, artificial rock reefs intended to provide shelter for lobsters and crayfish. The model assumes that the rocks are spherical and that each crevice is formed by the independent packing of three rocks. We use this model to derive the mean and variance of the rock size distribution needed to produce a given mean and variance for a crevice size distribution. These results are used as the initial estimates in an algorithm to choose the rock size distribution which comes closest to producing a crevice size distribution appropriate for the creation of new lobster or crayfish habitat.