Thanks for the feedback folks. Here's what I think I'm going to do.
1. Install the exterior piece and live with it. Only the mailman will know the cutout is not exactly the right size. Even he might not notice. He definitely wouldn't care.
2. Install the interior piece and see what it looks like. I could either
a. Paint the exposed door surface black to reduce the contrast with the
mail slot, and see how it looks.
b. Router just the edge of the interior with a roundover bit, to soften the
cutout appearance. Then paint the interior of the slot cavity black.
Potential problem here since I would be painting end grain.
3. I just thought of this. I could buy another door flap and install it on the inside. Not expensive in the big picture. Can't take money with you anyway.
If #1 and #2 don't work, I can use a router to open up the cutout. I would definitely clamp a guide to the door (door on sawhorses), and use a 1/2" bit.