You could also buy a "dry bag" and place t over the unit when needed. You can even modify it by cutting it down to size and attaching an elastic band to stretch and fit over unit to hold.
These bags are great to keep water out and can be used to keep things dry when you aren't using it on the sonar.
Amazon.com : The Friendly Swede 500D PVC Tarp Dry Bag (2 Pack), Yellow and Blue, 340 Oz (10 L) and 170 Oz (5 L) : Sports & Outdoors