Thank you all again for the deluge of tips & links!
This is getting seriously fun!

Quick update on my trials with copperclad (maybe just copperflashed)
metal-inert-gas continuous welding steelwi

- This stuff solders like heaven! Steel disperses heat less than
copper, and capillarity is amazing. Hot solder joints catch deep
between twisted wires.

- Steel is quite springy and retains shape. My spool is small (~5"
dia.) but if I release the wire it falls into 15" loops.

- I made a 20' dia. loop with 3+ rounds of this wire, and soldered it
at 6 spots all round. Very light and elastic.

You know where I'm heading. I would like to try a lightweight soldered
all-steel cage, and rustproof it with one of those steel-blackening
nitric acid paints.

As a trial, I'd do a short cage with two 20" rings, maybe 2' between
them, and 3' end cones, paint it all, and hang it out in the elements,
weight-loaded.

The INEVITABLE QUESTION
=== Do cage spreaders HAVE to be insulating? ===

Uneducated guess: as VHF/UHF discones are made out of either radials
or sheet metal, I'd say that shorting spacers should be OK.