Thanks, WTM. You got the ol' gray cells working here.
I bought a used Lowrance Elite 7Ti chartplotter recently (the guy left several years worth of fishing spots on it - yay) and I'm concerned about thieves. (actually, all those fishing spots won't do me much good - too far south, out of my range and too far offshore for my little skiff. Oh well)
The transducer has side scan, chirp, and I don't know what all else yet and it's gigantic compared to my old Garmin 240 Blue 'ducer. It's also very spendy and I know thieves will get a gleam in their eye. On another forum, I was told not to worry about it - with running cables and all, it's too much of a job to steal. Haw haw.
I don't want to drill holes in transom for it, so will mount it on a board epoxied to transom and run the cables up and over to an open frame console thing on the grab bar that I'm designing. Theft would be quick and simple.
I don't have a welder and in my rental house here, my shop is very limited for now so getting fancy isn't an option - and I have come up with some weird and wonderful ideas.
Going by your picture, I think I could mount a square tube, maybe 1" or 1 1/2" on the transom and find a matching piece that would be a good slip fit inside it to mount the transducer on. Slide it in from underneath and put a drop tab pin thru to lock it in place. I'll be checking it more carefully soon. Thanks for the thought.