If you do 2-1/2 inch of firebrick, 1/2" of cladding should be adequate. It is going to crack, but it is not visible and should remain structural as it pertains to the minimal thrust of the of oven.
I would like to see anything you have that shows anything different than 500f...it's always what I went on, and was told by a couple refractory engineers. But that doesn't mean their word is law, I just haven't seen a lot of trust put in portland for high temp applications...ones that are higher than wood ovens.Use the scratch refractory mortar for the cladding. I have made cubes and tested them in both the oven and bonfires, they do just fine. For laying the brick, however, Heatstop is worth the money. You can do 4" vermi/perlcrete then stucco over that for an igloo shape or as a monocoque shell if you do not want the extra cladding on the oven itself.
Some reading I have been doing leads me to believe that there has been a misunderstanding at what temperature portland cement reverts to it's constituent parts. I have had portland plant lab engineers tell me "500 degrees" is when portland cement begins to fail, but I did not clarify centigrade or Fahrenheit. Some of the recent articles I have read give the number as 500 centigrade.
That would be better, yes...17.25"-18". I would err closer to 10"-10.75 on the rise too. You don't want it too wide open...and you have a smaller oven than Bytor.rise would be somewhere between 10.75-11.5". In Bytors thread Tscar thought that a 16" wide opening was tight, so what's ideal? 18"
Take the card out of the camera and put it directly in your computer (if it has a card slot), or get a USB card adapter for your computer.....
I've finally found a camera but this one won't recognize the USB so I could take pics but they are on the camera and I have no idea how to get them on the computer. If I don't find one of the good cameras soon I'll have to buy a new one