Organic-inorganic hybrid perovskites have been intensively studied for their use in optoelectronic devices due to their utilization of low-cost, earth-abundant precursors that are solution-processed at low-temperatures into high-quality devices. Despite this progress, interdevice variability and long-term stability have prevented the widespread commercial adoption of perovskite devices, especially for high-energy photon detectors. Using methylammonium lead iodide perovskite single crystals grown via inverse-temperature crystallization, we demonstrate a facile solution-based technique to coat the single-crystalline bulk with a micrometer-scale thick surface layer comprised of a wider band gap two-dimensional Ruddlesden-Popper (RP) hybrid perovskite. The resulting perovskite room-temperature γ-ray detector devices exhibit greatly improved device yield and repeatability from run-to-run and device-to-device within a given processing run. With an energy resolution of under 15% (12.0 keV) for incident 81 keV photons, this solution-based technique resolves interdevice variability concerns and could pave the way for low-cost, scalable manufacturing of optoelectronic devices based on RP hybrid perovskite films.