The forming of the enclosure for the bomb is likely trivial compared with the other aspects. Anybody with a milling machine has probably been able to do it for the last 50 years. Now I can buy a CNC machine...so what. How about making the trigger, and creating and forming the HE to create exactly the right force to implode the fissile material.
The part that's not trivial is refining the plutonium a la Iran. Until somebody can 3D print 10,000 centrifuges and toms of Uranium Hexafluoride, Dr. LSU is still an idiot.
BTW you CAN buy a 3D milling machine for $1200 that can drill and print AR-15 lowers with no S/N. And it's legal too.