Fill an oddly shaped cavity in a model made by someone else

I’d like to fill this gap that was created - if I try to delete any of the faces, I get an error indicating that the hole cannot be filled. I’ve tried seemingly every function (but I am a newbie, so probably doing it wrong).
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Here’s another angle.
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if it looks familiar, it’s because this is a model of an apple watch (that someone else created). I’m just trying to fill it as if the faces around the watch band cavity continued along their existing pathways and met.