Derek Cohen (in Perth, Australia)
David, no doubt you do not need advice here, so I add this to contribute to the discussion.

I’ve done quite a few of these types of doors. This was the first time I posted a build (not flat panel, however) ....

http://www.inthewoodshop.com/Furniture/Buildtheframe.html

Sean Hughto also wrote a nice pictorial making similar doors: http://www.inthewoodshop.com/Furniture/Making%20a%20frame%20and%20panel%20door.html

Since then I have coped the corners as Bill describes, rather than just mitering them as above. Having said that, the mitres have remained tight over the years.

Regards from Perth

Derek