Your method looks like it would work quite well as long as there is no air gaps between the container and the hull.

I have seen/heard of many methods used accomplish a temporary thru hull transducer setup and they all really only differ in the type of container used to hold some water to transfer the signal.  Plumbers putty, Ziplocs, foam cutouts, food containers....lots of vessels doing the same thing. 

I haven't really created something new, just maybe my version or take on it.  The use of a 3D printer opens up more possibilities and instead of repurposing something to make it fit my needs, I can design something how I feel it should be.  This transducer well will only work with the two specific Lowrance transducers mentioned as it is designed around their specific shape and uses the bracket mounting hole for secure attachment.

Those downspouts are for rod protection I assume?  Do you just use a short section to protect the rod tip or are they longer?  Great idea and lightweight too.