Little Window sill challenge

 Window sills at the cabin are going to be spiffed up ceiling rafters.  They don't look too bad... from above:

But underneath, the drywall and plaster didn't quite get all the way up.  I was thinking about putting some other trim piece in, but that will start to look weird. 

So, instead, I dug out 1/2" of the underside:

and now it settles down pretty close to covering the crack.  This is the worse one: