I've never used the marine goop before, so I'm not sure what went wrong with the curing process. You could probably consider making one of the circular enclosures filled with water (and a lid on top) that some of the folks on the forum have made.
Humminbird made a kayak transducer mounting kit that works great for their transducers. I'm wondering if the non-DI kit would fit your Lowrance transducer. It's a clean setup where you affix the outer ring where the transducer sits in. Then you add the grease at the bottom and push the transducer down into it. A velco strap runs over the top to hold the transducer firmly in place. I believe that particular type of grease acts just like the grease they use for mammagrams and allows the signal to pass through without any interference.
Just throwing some suggestions out to you. My Humminbird transducer mount is pictured in the forums here:viewtopic.php?f=78&t=41489&hilit=transducer+kit
I put my transducer where you have the battery due to the smoothness of that area versus behind the mast post. The DI transducer is long and narrow, but the regular Humminbird transducer kit might work for your transducer.