Need Advice: How to Close out top of Tub surround

[Carpfisher]

The previous home onwer did a crappy job on an installation. I tried to find a "kit" to close out te top of the surround. at the cornerThe gap is about 0.5" See pix. I am open to suggestions!

 

[Stuff]

What about some non-expandable foam as a base and then some silicon caulk?