The standard response for repairs is usually Cascos Husfix, but after testing it on an area that needed filling, I think it's a terrible product compared to the alternatives. I would use Finja's handspackel Grov. The downside is that it is only sold in sacks, so you might have to choose something that's available in more convenient packaging if you don't have more to fix.
Yes, it might be overkill to buy a whole sack. I do have an entire wall waiting to be plastered and have three bags of weber gypsum plaster. I also have a small box of Bostik rep 'n fix. I guess either of them would work.
The suggestion you provided above seems good in the sense that it can be shaped and made nice during the drying time. Maybe it can also work with one of the two options I already have? Otherwise, I thought maybe I could use a small board as support to achieve a nice edge?
The only reasonable thing is to use something you have.....! Plaster may also work well if you can apply it in the thickness you need here without it collapsing.
