Ed-
I've been thinking about this too. So far I've decided that if I were to do it, I'd get a portable fish finder off of ebay for like 200 bucks then make a little platform for it to mount on which would have straps to attach to the tube. The fishin buddy comes in a couple models. Apparently the $150 model looks like it came from toys r us so if you go with the fishin buddy get the $200 dollar one.
-Brian