John Pagett G4YTJ
Notwithstanding Hans’ comment about using superglue, when I tried it in the past it set so quickly as the parts were placed together that it was impossible to get a snug fit because the set glue was too thick.
I eventually used silicone rubber, which had the advantage that with pressure the gap can be squeezed right down.
Admittedly this was with very flat faces of two transformer halves, but it worked very well on what was a commercial product.
The two parts stayed together well once the silicone had set.