You don’t need a universal milling machine to do this, or a 25k$ shop.
You need patience, a good set of calipers, the metal, a simple lathe, a rotary table (maybe 300$ one) tilted at a nearly 45 degree angle on a normal mill. You might be able to do this on a good drill press with the rotary table cutting the almost sphere with a fly cutter, but drill press isn’t meaning to take side thrust from cutting forces. It would probably work though, but for the play in the quill bearings on the drill press.
A lathe for 800-1000$, or less if you look, could get you the lathe needed. Mills are usually more, so get a beefy drill press. 50$ arbor press and 20$ heat gun both harbor freight.
You can do this if you have none of this with maybe 2k$ in decent old manual tools if you look hard.
Manual equipment is coming up really cheap now due to pandemic, its a good time to get into it.
Cheapest route- join your local hackerspace or makerspace- and use their equipment.
This technique he uses is called milling a sphere with an inward turned flycutter and rotary table. Don’t bother trying to find a universal milling machine (the kind where the table tilts like that), they are hard to find, and even harder to properly set up.
This is an awesome video, first time I’ve actually seen someone use this milling technique on video, it’s in 100 year old machining texts on my shelf, just never saw anyone actually do it!
