Ok, it's done good enough now.

I decided the screw should be threaded into the lower portion. The 3mm taps I have were too short, so I made a crude tap from a fresh screw. Sorry, no photos, but it is dead simple - just grind three flats making a tetrahedral point at the end of the screw. The casting is Zamak or some similar alloy, very nice cutting, so the made-tap bit right in and the threads were good enough. I actually made the tap twice, second time the flats were much more oblique so as to thread deeper into the hole.

Then same as before, cut the head off the screw and grind a slot in the end, put it in like a setscrew. After that some steel-filled epoxy, bit of work with files and then some dull silver paint: