Just an update on this topic. I was able to successfully install my echoMAP 50dv in my Oasis with the DownVu transducer INSIDE the hull.
First, the install. I loosely based my solution off this post taking the battery box idea and modifying it slightly.
I purchased a "cigarette lighter" style, 12-volt DC connector from Bass Pro Shops for about $20 and then drilled a 1-inch hold into the front of a Pelican 1120 case that I had purchased on Amazon (the orange color was cheaper) to install it. I then cut the Garmin 8-pin power connector before the attached fuse and installed some blade connectors for connecting to the battery. The other end of the power cable was spliced/soldered to the male end of the "cigarette lighter" connector. This photo was taken before I cleaned up the remaining six wires that come with the Garmin power cable.
The transducer was installed under the front seat, roughly in between the two Mirage Drives, using duct seal. The battery box was placed just behind the transducer, to be bungied around the front seat scuppers. NOTE: It gets pretty tight in here once the transducer is installed. I found this out the hard way AFTER I installed the transducer. However, it is still possible to move the battery box in and out of the hull with the transducer attached.
To attach the actual fish finder, I screwed a YakAttack GearTrac 4" to the upper lip of the kayak, just ahead of the rudder deploy control cable. To attach the unit to the track I used the following:
Once everything was installed and connected I hit the water for some evening fishing. I poured about a 1/2 cup of lake water on to the transducer and duct seal, however, the sonar would not read properly:
After some on-the-water troubleshooting, I found that I had sealed around the transducer TOO well not allowing any water to get down to the tranducer itself. I shut off the fish finder, manipulated the duct seal a bit more which allowed for a tad more space between the unit and the sealant, poured a bit more water on the combo, started the fish finder back up and viola!
Initial observations reveal very good reception through the Hobie hull. The deepest water that I encountered on the first outing was around 30 feet which still yielded great structure detail and fish returns. I am new to fish finders overall so I will admit that I do not have much of a basis for comparison. However, I will continue tweaking the install and setups on the unit and try and update this post as needed.