I'm not familiar with the Ericson 29, so some of this may not apply.
If your boat hull is cored in that location, then you'd not be able to place the depthfinder there.
There might be an air bubble between the fiberglass and petro jelly, or within the fiberglass itself. That would prevent the transducer from working as expected.
Also, I've not heard of the petro jelly being used as a test bed. I'm not sure how effective it would be. Have you tried the plastic bag of water approach?