Yeah got white Lowrance clip on cover to protect the screen (cheapest i got 7" was on ebay) but also bought a clear glass screensaver.
Wouldn't stress about complete cover as PD is correct...hence why there isn't anything available
But I did see open tied up boat with fishfinder with a clear food tub slotted over the top......I guess to keep the seagull sh*t off as much as anything.
Another cheapo option is some of the tablet/phone so called waterproof pouches which would work.