Why use a portable depthfinder? Almost all depthfinders haveor you can get bracket to mount to your trolling motor. You might need to run 12 volt leads up to the front of your boat but in most boats that is fairly simple. As an example go to the following
http://www.eaglegps.com/Downloads/Ma...651_091902.pdf
Look on page 8 of the manual and it shows the mounting. I refer you to Eagle because that is what I have on my boat, but H-bird and Garmin are similar to Eagle/Lowrance. Would make it a lot simpler than having to have extra 'A' batteries all the time. Simply run the cable from transducer up the shaft of the T-motor and attach with either electrical tape or cable ties. You can place depth finder where ever it is convenient and viewable.
Hope this answers your question and does not confuse you to much.