Who makes it doesn’t make it “have graces” or anything like that, it’s the blessing that is put on it, just the same as a rosary or any other sacramental–if you struck your own Miraculous Medals at home they’d still be Miraculous Medals. I bought some wool yarn a while back to try to make some of my own but never quite got around to it. Have at it, though.
It’s important to remember that scapulars and other sacramentals are not magic, but are simply a means of disposing us to the divine grace which comes to us from the sacraments and from God himself. So the scapular itself is just a means of preparing us for grace, as would any rosary, medal, or anything like that. This is why the maker of something like that isn’t important–if I make sacred art that reminds me of God and the saints, it’s a sacramental, it doesn’t have to come from anyone special or a special factory or a convent or anything like that.
-ACEGC