A quick thought that occured to me here:
With your setup, if you really do get hooked on ovens while building yours (like a lot of us have here) you can always plan to build a smaller (round?)one at a later date somewhere else.
You should be able to get lots of support for that, especially when the congregation sees how well these ovens work... now there's a perspective I'd like to have!
Good luck with your build! Sounds as if this will be really interesting to watch, what with all the expertise you have to back you up.
Frances