Here's what you need:
-1x Cardboard duct tape core
-8-12x Rubber bands
-6ft Duct tape
Space half the rubber bands evenly around the core, then tightly wrap with duct tape. Add the rest of the rubber bands and wrap again. You can then tape it directly to a mic stand, or attach it to a standard vocal mic clip for pivot control.
I was never able to find my missing shockmount and continued to use the MacGyver prototype on a regular basis for a year before I had to make a new one. (The rubber bands will lose tension and the cardboard will lose firmness over time.) Still though, it's pretty hard to justify spending $15-$50 on a factory shockmount when you can make a universal one for about fifty cents.
I have not yet used it with any tube mic, the heat may affect the rubber bands. When trying it with any new mic, make sure it feels tight and secure in the rubber bands. Heavier mics will require more bands.