Eric in North TX wrote:
A large
protractor, or is there a device that fits inside interior angles (such
as inside the door frame) to measure them?

I have a device, not sure what it is called, it has a wooden handle
with a thin metal blade that folds out like a pocket knife that is
handy for finding angles, it is a pretty common tool, not hard to find.
You may encounter another problem, once you find the angles, the cuts
may not have the same face length. Unless it is horribly out of square,
it may be better to just cut the miters 45 and at least have part of it
right.

The correct name is "bevel gauge". I call it an "angle guesser"

You fit it too the corner, tighten the clamp. Then all you have to do
(hah!) is bisect that angle. How to do that? I could at one time on a
planet long ago and far away explain it using geometry. Real world is
"Take a guess then make repeated cuts" with your your saw changing the
angle slightly each time until you get it right. Use scrap wood for
the tests.

Harry K