There used to be a portable fishfinder that came in it's own hard cased box with batteries and eveything. The transducer was mounted with a suction cup that worked quite well for kayaks and aluminium boats and whatever. It was a good little unit. I had one.

Not sure if they still make units like that, but the suction cup idea might still work on a typical trandsducer. Either that or a pvc pipe with a clamp attached would probably work.

I'd check out the float tube forum pics for rigging ideas. Those guys have some sweet set ups that would work on just about anything.