The US2 is 2D sonar ONLY.
If you want Down Imaging, a Down Imaging transducer has to be used.
If you are only going to use 2D sonar you are wasting your money purchasing a Helix 9 DI GPS.
For trolling motor use, do not adapt the transom DI transducer. Use the proper transducer for the application.
The Helix 9 DI GPS has 83/200 kHz capability so the proper trolling motor PUCK is the XTM 9 WIDE DI 20 T.
The trolling motor PUCKS have extra internal shielding that the transom transducer does not have. The shielding helps with possible RFI from the trolling motor.
Bottom line is, use the correct parts for the application.