Cutting a new replacement and keeping if longer is a good suggtestion if you're really bent on getting a perfect miter joint.
I'd Try sanding the part in the picture on the bottom a little (where the joint is perfect), sand, check, sand check.....keep doing this until the pieces come better aligned. This should get is closer.
Wood putty is a captenters best bud. A little smeard in there and sanded, then painted and no one but you will know it wasn't PERFECT!
Other than that, looks pretty sweet. I love your CPO as well as what I saw in the Project Announcement thread.....