That plastic bracket is just the part that came with transducer that goes into the metal transom bracket. You should already have that. Unless they didn’t give it to you. I have never seen a bracket like yours on a transducer before. Here is picture of complete ashy. The part that goes to transducer is on right. It doesn’t fit perfect on motor housing but mine hasn’t moved yet. But before purchasing something I’d sure try some rubber to isolate it to see if that helps.
https://www.walmart.com/ip/Humminbir...CABEgKGEfD_BwE
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Charlie Weaver USN/ENC 1965-1979