Installing a VW 1.9 TDI glow plug in a Listeroid CS COV plug can be done on a cheap Central Machinery lathe in a single setup.
I Chucked the combustion chamber end, which is cylindrical. While just lightly fit in the chuck, I ran the 'dead center' into the center hole in the square end, effectively reentering enough for a glow plug.
I Drilled thru 1/4" for the glow plug tip, then bigger holes for the glow plug body and seal, 23/64" tap drill size for the 10x1.0 mm tap, and 3/4" recess for the head of the glow plug and to fit a deep well socket to install/remove the glow plug.
I used a boring bar to spot face the 3/4" bore's tapered bottom from the drill.
I installed the 10x1.0mm tap in the drill chuck, and turned the work by hand, getting a perfectly straight thread. The glow plug tip is centered in the plug face.
My COV plug looks like the two left ones in XYZR's photo above.
VW 1.9 TDI glow plugs are a bit shorter that depicted on XYZR's pictures, but I can't see that it will lead to any problems.
The NGK 5909, also known as Y-732j is a self regulating unit that can tolerate manual operation and long 'on' times. I used a generous gap around the tip to prevent seizure from swelling after the plugs get old, and hope the space will assist with cold starts. The NGK plug was locally available for $10, so I gave it a chance. If it's got a short life, I can pay more for a Bosch or something.