Material ID Pass Generator does exactly as it's titled! It'll quickly generate a material ID pass following your render resolution.

A Material ID Pass is incredibly useful when editing your renders in external software. It allows for super easy selection of objects by material.

-Demo-

v1.1 Coming Soon! v1.1 will introduce a save button in the Image Editor Window. The button will save the generated pass in the recommended file type & color space(EXR DWAA & sRGB Standard).