They are very simple but the holes are small. Normally they drill a small hole up to maybe 1/8" in the body of a SS bolt then perpendicular to that hold drill another hole around .024" in diam(.020 to .026" depending on shot size) from the side of the hex in to the larger hole. Saw some with some double holes that id love to have for larger production. Also the exit hole can be beveled at about 15 degrees but i dont think it mandatory.
I would think that the easiest way to make them would be to find someone to EDM the holes in them. The number 9 drippers have a .020" hole, at least that's what mine measure out at. Also, Magma sells the drippers and are touted to be better. For as little as they sell for I'd be hard pressed to try and justify the work involved in producing a set. Here's my set up -