=== Do spreaders HAVE to be insulating? ===

YES for sure, for sure. The cage should be really folded dipole, one "wire"
going up (support, and grounded at the bottom) the other wire on the outside
(cage) coming down and fed against the ground or radials. Unless you just want
to make it fat radiator, then it doesn't matter.

Uh? AFAIK, a cage is not a folded monopole.

The "inside" vertical conductor would be shielded by the cage itself
('cept for its very bottom tip, which is grounded) and would not
radiate ('cept locally, within the cage, hence autocanceled). Apart
from discones I also remember seeing multi-kW 1940's TV dipole arrays
in which arms were huge, bulbous surfaces without slots.

This is getting *seriously* complicated.

One should be all thumbs to afford enough rules-of-thumb.

Some spreaders are indeed conductive.
But perhaps the fastening system acts as insulator.
http://www.stockroom.com/j030plus.htm
http://www.stockroom.com/j032plus.htm
You never know.