佳作 is mostly used for artistic works, for instance movies, novels, music, paintings, pottery, etc., not tools such like umbrellas.
As OoTmaster-san pointed out, the kanji determines the meaning of the word. "Kasa kasaku" even can mean "I make only a few umbrellas" without kanji. I recommend using a brand name with kanji, not just romaji.
In addition, how the words are connected (the word order or the position of the possessive particle の) is also the key. It would be better to make a word in Japanese first and then change it to romaji (e.g. 極上傘 Gokujōgasa "excellent umbrellas"), not just simply to connect those words.