A diverter test cell can include an enclosure having a body and a cover removably coupled to the body, where the body forms a cavity that is enclosed by the cover, where the body includes an inlet port and an outlet port in communication with the cavity. The diverter test cell can also include an insert removably disposed within the cavity, where the insert has a channel that forms continuously from a first end to a second end of the insert, where the channel has first width at the first end and a second width at the second end, where the first width is less than the second width, where the first end of the insert is adjacent to the inlet port of the enclosure, and where the second end of the insert is adjacent to the outlet port of the enclosure.