The wiring for the lighting is done by electrician, nothing to do with the plaster ceiling chap, although your contractor will make a margin for pulling the whole thing together for you.

Lots of folk sometime forget plaster ceiling drive up lighting cost because you need to re do the lighting wiring. Those lighting point wiring will be expensive but the down light itself is cheap.

BTW, doing those L box will increase house maint work. Spider and dust loves those box.

AudiA6

Square down light with glass cover is only RM22 (6in) with 1 GE 18w bulb. RM32 for double bulb.

Dirt cheap nowaday!