If you are considering the 798 model which only has 455 khz for SI, there is a trolling motor puck for SI and 2D that mounts just like any other 2D trolling motor puck ( XTM 9 SI 180 T ). It is a lot sturdier than the bracket that RGecy posted.
You can exchange the supplied transom transducer thru Humminbird for that type if it applies. Just don't open the transducer package in the box so you can return all the stuff included with it.
Ask the supplier if they will exchange it before you take it or have it shipped.
With the HDSI transducer for the other models, that bracket will work or for a secure installation and protection, the Transducer Shield and Saver is a better mount.