the majority of the xducers are mounted on the TM using the mfgr's TM mount which usuall consists of some type of metal/plastic bracket and then mounted to the lower motor unit (blade end) using a large worm type clamp that goes around the lower motor unit. You just need to be careful how you mount it in terms of routing the xducer cable in a way to avoid it binding up or getting pinched when deploying/stowing the TM. This is for a simple 2d FF or DI. Any movement of the TM when controling the boat affects the picture to a limited degree. If a SI unit, I've read where it really messes up the picture, most mount the SI xducer on the transom from what I've read, but options are available.
Look in the Electronics forum and you'll find lots of info on how various members have done it.
GO BIG ORANGE !
I meant to behave, but there were just way too many other options available at the time.