You could get a joinery firm to make you some shaped skirting board up to fit the bay....otherwise the only other way I can think of is lots of short lengths side by side to make an 'edged' semi circle...
Whats the plaster like your fixing to? if its in good order then depending on which route you choose I would drill the skirting to enable accurate marking of the wall, then use plaster plugs and screw into them.
Countersink the screws into the skirting and then use wood filler over the heads when your done...
lot of work tho doing it in sections.
I would much rather get a piece bent to the curve like this
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