I think I'm the same situation as you - I'd like to have mine setup to peak around the corner from the side.
imgur.com/a/jzyUcYrOnly other thing I've seen is the Railblaza system -
www.whitworths.com.au/railblaza-fishfinder-mount-r-lock-r but you might need a little extension arm to make it work, and a starport to match. Adding all those together comes to about $75.
Happy to hear anyone elses ideas though - I had thought TV mount, but doubt you'd get one in stainless and probably overkill for the size of chartplotter (compared to modern tvs!).