Metal Artifact Reduction (MAR) is required for orthopedic imaging near implants. Novel MAR techniques have been developed in the past decade, creating a need for quantitative evaluation of the effectiveness of geometric distortion correction. We have developed a 3D-printed modular conformal grid phantom, consisting of a grid of regularly spaced spherical markers. This phantom provides a measure of inherent field inhomogeneity, and contains a conformal cavity in which a metal object can be embedded. This approach provides a means to characterize through-plane and in-plane artifacts across a 3D volume, facilitating testing and validation of novel MAR techniques during development.