Reconstructive spectroscopy in the mid-infrared (MIR) range is an attractive method for miniaturizing MIR spectrometers. Although detectors with a sharp responsivity spectrum and a high dynamic range are required, there remains room for improvement. This article reports on a set of MIR photodetectors that satisfy these requirements with aluminum-covered narrow trench gratings. We experimentally found the optimal grating design, by which the responsivity changed 0.75% of the maximum value per nanometer of wavelength, and the minimum/maximum ratio was 8.7% in the wavelength range of 2.5-3.7 µm. A detector set of this optimal design was capable of accurate and robust reconstruction against measurement errors. This simple design structure based on standard materials will assist in realizing miniature MIR spectrometers.