I saw/read an article or advert for polystyrene tubes that were shaped for 4x4 KO/ 4x2 KO boxes and round boxes ,and came in a 1 meter length .
You could then insert into the wall box and cut the depth to suit which then prevents plaster or concrete ingress.
I cannot remember who the supplier was and as per normal when you want to find the article google does not cooperate.

Has anybody used the tubes or know where they come from