... is that the E5DSI unit transducer you're talking about ?? If so, it should have come with transom mount bracket & parts.

IF it's another unit, with a puck style transducer, then you might have problems if you want to mount it on the outside/back of the transom. Might be better to mount it inside, "shoot thru the hull" style.

My console mounted HB 2d unit is mounted "shoot thru the hull" .... works fine. (it came that way with the boat)
You can see the HB unit in this picture, just to the right of the steering wheel. The puck Xducer is epoxied onto the bilge flooring.




If it is a puck style xducer, and a new unit ... see if the mfg will trade you. If it's a used one ... see if you can sell it on the Classifieds, or find someone to trade with you.

... cp