The curved cornice has nothing behind it. Only the top and bottom are attached to anything. This made filling the gap a seemingly impossible job, as there was no there there to attach anything to. There was just the thin edge of metal.
After some pondering, I realized spray foam would do the trick. It would attach to the thin tin edge, but more importantly, it would ooze BEHIND the metal and attach, and attach to the wood wall.
After the foam dried, I carefully shaved it smooth with an extra sharp knife, and then, because the foam is porous, I used exterior caulk as a sealant over the foam. I was extra fussy with this step.
The results thrill me.
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