Yup, Playdoh, or other such child's modelling clay.
Wanted to test fitting my transom mount transducer inside the hull
and shooting through. Having read about creating a fluid filled wet well to put the transducer in, I didn't want to go this route
until I had determined a clean location for best signal. With the idea being that the transducer needs "solid" contact all the through the hull
I took a lump of Playdoh and made a thick bed
to push the transducer into. The unit is a raymarine
WiFish BTW. Worked great, which surprised me. I need to pull the unit out and hang it over the side on a pole as a "gold standard" test for comparison, but in the meantime the Playdoh is good enough for testing purposes. Hope this helps, definately not intended as a long term installation