The SOFA/ambiX binaural rendering (SABRE) toolkit is an open-source collection of MATLAB functions for generating custom ambisonics-to-binaural decoders for Matthias Kronlachner's ambiX binaural plug-in. Databases of head-related transfer functions (HRTFs) are becoming widely available in the recently-standardized "SOFA format" (spatially-oriented format for acoustics), but there is currently no direct way to use custom HRTFs with the ambiX binaural plug-in. This toolkit enables the user to generate custom binaural rendering configurations for the plug-in from any SOFA-formatted HRTFs or to add HRTFs to an existing ambisonics decoder. Also implemented in the toolkit are several methods of HRTF interpolation and equalization. The mathematical conventions, ambisonics theory, and signal processing implemented in the toolkit are described in this paper, and the toolkit is freely-available on GitHub.