There's various ones available already. Not saying a home-made one is bad, but I got a couple at a shop in Canada a year or so ago, and they sprayed wide and pretty clean. Interesting caps. Here's a pic of what they look like...
You can also find calligraphy caps of various spray widths on lots of other products. Some spray varnishes and automotive paints come with them. I recall Rusto used to make 15oz cans called Big Shots that also came stock with calligraphy/fan spray nozzles. Really good paint, to boot. I always used it with rusto fats, though.
The danvern fan spray caps (the annoying blue ones with the touch button action that won't allow the can to take any other kind of cap) are probably the best if you're looking to do fan tip tags. Light pressure and pretty wide spray. A lot of the paints that come with those kind of caps are very good, too.