I'd turn them from bar stock, or else try metal spinning which is often used to make velocity stacks. This guy has metal spinning info on his site and will sell a CDROM:
http://www.jamesriser.com/Machinery/Machinery.html
Do you just need to make one or a few of these, or do you plan to make a bunch? If you want to make a bunch, CNC converting a 9x20 (or I prefer the 8x18 Lathemasters) would crank 'em out of bar stock lickety split.
Best,
BW