In the end, I fitted an offcut of cement board, then a layer of readymix mortar, then a thin layer of polyfilla (because it’s white), and then a couple of coats of white gloss paint to make it a bit more waterproof. Careful positioning - and luck with the plumbing - has resulted in nothing visible from the room and just a few mm of paint visible from when you’re actually in the shower:
View attachment 384836
View attachment 384837
The main difficulty was that the four tiles were not perfectly level - unsurprisingly, since they were fitted around the shower so they could not have been matched up with each other. That has resulted in the cover not being perfectly flush everywhere.
P.S. it seems to be exactly the same shower that
@pete01 has in post #2!