I have been searching for a generic case for some of my melodicas and found this thread. Don’t know if anyone is interested by found this
http://global.rakuten.com/en/store/kurosawastore/item/suz-mp-5100b/
I haven’t ordered it yet and can’t speak to it’s effectiveness but it appears that the Japanese are the only people making generic cases since melodica is so well accepted in that country as a entry-level instrument for school children.