In time comes progress!
Thanks to a posting I came across I now have a non-permanent thru-hull solution for using my portable fish finder on the kayak. It's so simple it's brilliant and of course I thought "DUH... Why didn't I think of that before?!"
I installed a wet transducer well inside the hull. Using an inexpensive 4 1/5" PVC straight pipe joint with the end shaped on a belt sander to fit the contour of the hull I "gooped" it in place. The transducer is lowered into the well, some water is poured into the well and a piece of pool noodle or sponge is pressed in on top to hold the transducer in place. The only thing remaining was to cut a small access slot in the forward hatch cover to accomodate passage of the transducer cable from the portable fish finder that's secured on the front deck. Here's a photo, though it isn't as clear as I would have hoped for it might help to understand what I've said.