My transducer arrangement has the unit against the front bulkhead in my Prism.  However, I didn't' put the transducer into a dollop of silicone.  Instead, I took a section of plastic downspout and cut it down to about 4 inches after trimming the bottom edges to conform to the bottom of the hull.  I then used silicone to make a water tight "cup" of the section of downspout to hold my transducer.  When I get ready to put my depth finder into action, I drop the transducer into the cup and add water until the transducer is covered.  I cut a sponge for this purpose and I cut it somewhat larger than the cup so that after use,  it is pushed down into the cup to help anchor the transducer.  The transducer will generally stay in place on all but the longest portages, but I also added a few plastic clips just under the gunwale from the location I use to mount the main unit to the bulkhead.  These clips keep the cable secure and out of the way for loading/unloading packs.  Secondarily, the clip just above the cup on the bulkhead adds additional security for the transducer should it fall out of the cup on a portage.

Of course, you have to add water to the cup after each portage if you want to fish the next lake.

BTW:  I also cut/spliced the transducer cable to eliminate much of the excess cable.

dd