Hi Dave,
We have not tried the attachment method you speak of, however we do have 2 different types of portable transducer holders that attach to our canoe (and also on rental row boats) by means of a C clamp. They are aluminum and adjustable to depth. Worked well for us and our portable Humminbird. I cannot remember the brand name but I think they may still be available thru Bass Pro and also Cabela's catalogs. Hope this helps.