IF the hull pad does not go all the way left and right of the recessed transom, put the transducer here:
black box is the mount, yellow oval is the transducer.
It it does, put it on the left side of the hull drain opposite of the hull fitting seen in the photo.
You will still have to trim out some in the example when scanning at idle speed.
Humminbird Helix 12 SI, 1198c SI, 1198c SI, ONIX 10 SI NT, 360, RC-1