This happened to one of my fins a couple months ago;
never fixed a ding/box like this before but had a go and it came out like this;
This is before I sanded off the hotcoat. Reckon it was easier than I had made it in my head, looks like your fix in the top pic is better than mine though
Have seen FCS boxes trashed and torn chunks of foam out before though, lokbox was pretty clean and easy to repair. Didn't lose a fin either which was nice!