Can't actually be of any real help, but I've wondered the same thing as I have a Raymarine/Airmar in hull transducer that I presume is very similar to yours.
I filled it with Mineral oil, and got the top on good and tight, however the oil does still appear to evaporate or leak out somehow.
In theory any oil should do, as long as the signal is transmitted to the hull rather than having a pocket of air under it.
I think the reason mineral oil is used is because it's so inert, and doesn't contain anything that will attack the transducer, but there must be something a little less viscous.
At one stage I thought about using some kind of two pack jelly to fill it that you pour in liquid and allow to set, but didn't know how to get hold of any Ballistics jelly, and couldn't think of anything else that wouldn't glue everything together solid, even if you did coat everything with a release agent before hand.
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