One more thing, there are two ONIX SI transducer versions. The mount pictured is for the improved version (2).
The original is more rounded where you insert the transducer.
The "improvement" changed the shape of the trailing end to a larger hole to fill the inside during mfg.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, 360, RC-1