G'day MW, NK and JFB,
we're going for cheap right?
Scrunched up paper jammed in the cavity followed by the cheapest setting compound you can get, then some sort of thick black paint or other compound you can apply by brush, mini roller or pressure pack.
It shouldn't be this arduous. I think common cement was never destined to work in hindsight.
Whatever compound it is, it must be considered, cheap and flex a bit too, not be rigid.
Maybe tin donuts attached by screws??
