How would you fill this awkward angle/gap under the stairs?

[DoNotUseFullName]

I boarded under the stairs and have a ~20mm gap where the wall meets the underside of the stairs. How best to fill? It's my first time jointing and feel I'd make a hames of it considering the tight angle.

Maybe a small strip of coving? Any other ideas?

 

[Wayners]

PVA in gap. Got any wood glue? Let it dry. Fill best you can with powder filler. Let that dry.
Then wood quadrant stuck on with polymer grab.
Emulsion.

I always think its best to fill gap prior to quadrant.

 

[DoNotUseFullName]

Thanks. You wouldn't just use joint filler?

 

[Wayners]

If you're asking how I'd do it.
I would PVA then fill in two hits with two mixes 45mins apart. Id use tx-110 powdered filler.
As the second mix was setting I would dampen down white dry lining paper tape and fold along the crease. Set the paper tape along joint in the wet filler.
Make good.
Sand
Paint.
If you fancy giving that ago.


My original method was easier. Maybe not what you wanted to do though

 

[DoNotUseFullName]

Food for thought, thanks!