I decided that I wanted a brick archway to help protect the opening, so built a sand former and covered with newspaper. Then built an archway over the sand former using some oven mud mix as mortar and driveway mono blocks that one of my neighbours had left over. I used a sheet of plywood and a brush handle proped against my decking to keep the sand former firmly in place.
I have left a 5" hole between the arch and the dome as I intend to add a chimney over the next week or two.
I fired the oven up for its first cook last week and found it took well over 2 hours to get to a decent temp and even that only allowed 5 min cooks. I'm hoping the insulation layer will allow it to retain much more heat resulting in quicker start ups and longer retention of heat once cooking commences. Would a firing door also help?
Firing has been good with only minimal cosmetic cracks that only showed up on my first bake, you can see them on the final pic above where the chimney will be, this was when I loaded the oven with wood to the point the oven roof was reading 'high' on the infared which has a max temp of about 1050!
You'll need several good fires before you get full performance. There's a lot of retained moisture that needs driving out, and then after each winter (if you don't use it) you'll need to season the oven again to drive out any damp. The moisture means that the heat doesn't hold as the water acts as a medium to draw the heat out. It'll get better the more you use it.