That oven looks so so good! Well done. Do you have a thread anywhere for your build on the oven? I'm interested to know how thick you built the hearth and how much insulation you used underneath as well?
I don't on this forum but can give you a run down. If you want more specifics, just PM me and I can walk you through what I did right, and what I had to correct as I went. I built the CMU full base, spanning slab w/ rebar, etc.
After putting up a simple square foundation of CMU's, I filled every other hole with rebar and QuickCrete. Then I built a simple form out of 2x6's for the sides, and used concrete board for the bottom support of the slab, and used metal studs for support to keep it from bowing. I then used extensive amount of rebar throughout, and poured QuickCrete (long evening!) for the slab. I purposely underpoured the slab to leave the surface rough and under top grade. This was about my only added thoughts to the basic process. Instead of trusting myself to do good finish work, I left it short of top grade. Then I bought a couple bags of self-leveling concrete. I mixed this the next day to a slurry, poured it over the slab and - wow! - it was like glass, and perfectly level. My hearth was very happy!
Prior to laying the hearth, I used 5" of ceramic fiber board insulation rated to 2000 degrees, cut to about 5" outside of my hearth size. It went down, then my hearth was laid on a bed of notched troweled silica sand. The leveling process was arduous, but it came out pretty nice. From there, I laid on the monster chunks of refractory (FornoBravo Casa2G100). I joined these from the exterior only with 100 pounds of refractory mortar. Then set on the obscenely expensive 8" insulated stainless steel chimney and used fireplace caulk to seal.
On top of that came about 8" of ceramic fiber blanket. I ended up leaving it alone after that, and not adding any vermiculite. I didn't want the blanket compressed, and figured I'd try the vermiculite only if I didn't have good heat retention - doesn't look like I'll need it as it was averaging 350 degrees 24 hours after the last flame went out on it's first full fire. And that was without an insulated door.
From there, I framed it with metal studs and put on concrete board to wrap it and prep it for the finish work - which I paid someone to do so I didn't butcher it! The final piece was having the matching granite top and hearth placed.
There are obviously 100 other little things that have to be done between all those steps, but that's the "big picture". The best thing I did was render it many times in a simple drawing application on my mac, with dimensions. At each stage, i had renderings next to me to use. This was invaluable in determining layouts, and space for studs, chimney, etc. If you have specifics you want to know, just message me, I'd be happy to help out so you don't have any of the frustrations I encountered - which luckily were few for a newbie!