In order to clamp the tube in a V-block, the clamp must position the tube’s centerline so the envelope will clear the V groove, thusly:
The clamp now extends into the V-block and surrounds the entire Bakelite tube base:
The little divot captures the clamp screw and the slot lets the whole affair compress just enough to firmly squeeze the entire tube base.
The tube data table now includes columns for the envelope OD and the base OD, although only the 0D3 (and similar) Octal tubes in my collection have a bulging envelope and a smaller base. You can build clamps for cylindrical glass tubes if you like; I don’t vouch for the accuracy of the table contents.
For whatever it’s worth, the 6SN7GTB tube I started with has a 32 mm Bakelite base and the 0D3 tube has a 29 mm base. That should probably justify two separate entries in the table, but I’m making this up as I go along.
The OpenSCAD source code as a GitHub Gist: