Plumbers putty works better in my opinion
Duct seal is plumbers putty. I used that method for ahwile. It does work fine but it can become a mess. I finally broke down and paid $60 for a ram mount transducer arm kit from Hook1. Best money I have ever spent rigging my boat(besides the ff itself of course) Puts the transducer directly in the water (accurate readings for everything temp and depth) and it is easily tucked out of the way or removed if I don't need it.
I'm just speaking from experience. Ive gone the cheap route with the duct seal and it worked fine for awhile but it just wasn't cutting it. If you can spare the $60 I highly reccomend getting one of those transducer deployment arm kits. Mine mounts with a ram ball and swings easliy over the side or up and out of the way. love it!