Don't try to make one. lol I'll show you a prototype of an at the gun injector I built to reduce the sputtering I get from the x jet. It caused more problems than it solved.
Buy the x jet. It's costs 1/3 of the money you'd make from one small job if you're pricing right. They've made a good product. You're paying them and their engineers for hours of trial and error and multiple prototypes.
If you wanna modify to reduce sputter I'll find you a video on YouTube of a guy that's done it, tried it, made it better. Buy it first and let me know when you get it and I'll track down the video.