Deck Filler Cap

I'm trying to find just the plastic cap for the water tank deck filler on my Cal 31. The threads are 1-3/4 inch in diameter but I can't find anything indicating the manufacturer or model number. Not having any luck finding 1-3/4" caps via Google. Any help would be greatly appreciated. Thanks. John Stacklyn Cal 31 #33 "Sol Survivor" Shell Point, FL