Basic qu

[Sks87]

Left with these 2 holes from a tall radiator that was removed.
It’s quite thin. Do I need to put plasterboard down and secure it with wood (though doesn’t seem enough space) or can I just fill it with plaster?
Or something else?

Thanks

 

[Sks87]

 

[opps]

Punch the plugs below the surface and use filler. Then sand the filler flat.