Then I study a simple resolution and which should work correctly.
As I'm sure PEHD lets through waves, I sharpened in an end the same form of the plate of protection Hobie..
Then I embedded well the sonde with the foam high specific gravity so that it does not move in its accommodation, but a little lower than the first try..
Then I heated up with a thermic warmer, PEHD took the form of the transducer:
Then I screwed on screws thoroughly.
So the sonde is protected, both sides are clear and water circulates freely.
Have to see on water but I trust about it...