Humminbird sells a neoprene cover for the Helix 5 series which sells for $34.95 plus shipping. It is available from other vendors for a little less but while wandering around the Cabela's in Bowling Green I found a Cabela's cover for less than $7.00 - Its logo says Cabela's rather than Humminbird but for the difference in price I will accept that. It is called the "H5 Sonar Screen Cover" .

Cabelas-Sonar-Screen-Cover-Humminbird-Helix-Series

Select the 5" model in the drop down.