If you truly want the benefits of a flasher in open water you have to think like you do in the winter. Mounting the transducer to your transom or any other part of the boat totally defeats the purpose. That is to be able to see your lure in real time.
What you need to do is run the transducer cable along a boom away from the boat. Attached the cable to a hockey stick that is placed in a rod holder. You can adjust the length of the boom (hockey stick) and the depth of the transducer.
The whole premise is that you're using a 6 foot+ long rod therefore you want the rod tip far from the boat and be able to control the placement of the lure. Which is in the sonar cone.
And you want to be sitting comfortably, not having to lean over or sitting back to keep the rod tip just off the boat where a transom mounted transducer would be.
For the record, I've been doing the above for about 6 years. Works likitty split!