Through the study of existing lightweight cryptographic algorithms, we suggest a number of design guidelines for creating new sponge-based extendable-output functions for use in resource-constrained environments. While several such algorithms exist, some knowledge that can be generalized from studying them in aggregate has not previously been presented. The developed guidelines include consideration of the round function width, the number of rounds, the selection of round constants, the rate, the linear and non-linear layers, and the required security claim. The result of these guidelines is a set of recommendations for the design of sponge-based extendable-output functions that should allow correctly balanced security and performance in environments where compute power, available memory, and battery life may all be limited. These recommendations could be used to help design purpose-built implementations for various wireless or mobile systems.