Buy a cheap cast polystyrene ceiling rose from Amazon.
Run a circular mould around it to make it more decorative.
If you have to cut a profile for the running mould, try and pick up the same molding member that you have in the room on the architrave or skirting.
If you only need one then, just fit and fit the circular mould or if you need more then make a cast with cold pour,
I would set it out on a circular plasterboard to give you a bit of depth and thickness for casting.