The metal frame came from an old Flamefast bronze furnace. It is standing on breeze blocks to raise the height to a better working place and to provide a storage space underneath.
A layer of lightweight breeze blocks cover the frame. Down each side a groove has been cut out for the arch bricks to sit in, so that the sideways arch thrust is taken by the blocks and the metal frame.
The main arch is made of bricks left over from the chimney of Bill's wood fuelled pottery kiln (Ibstock Class B Engineer Bricks - with 3 holes), proven to cope with flames leaping past them!
They were trimmed down a bit into wedge shaped arch sections, using an angle grinder (and lots of disks!).
This image shows a mock-up attempt at checking the arch configuration, supported by an assortment of blocks and slates etc.
When happy with the shape, a template was made of the curve of the arch, which was then cut out from plywood - see below.  

The mock-up was removed and the breeze block layer was covered with overlapping layers of aluminium foil.

The foil was then covered with a layer of dense bricks from electric night storage heaters. These are very dense and smooth, so make a great base for the oven. They store heat energy which is released during the cooking period, after a fire has been burnt on top of them.
An arch former was constructed, using the profile drawn from the mocked up brick arch. Two pieces of plywood supported 5 cross battens on which hardboard was nailed. 4 old wooden tapered table legs were placed under the former to support it at the correct height - when finished these are knocked apart and the former drops down and can be pulled out.

The arch bricks were laid in staggered courses on the hardboard, some having been cut in half with a bolster chisel. Because they had not been cut to a complete taper there is a gap between them at the top. This gap was filled with clay shims, which were a by product of the wood fuelled kiln firing - in that kiln the pots are raised off the shelves (to stop them sticking) by placing each pot on 3 clay filled sea-shells, during the firing the clay gets fired to a hard ceramic and the shell is burnt to quicklime, which then crumbles away to release the pot, leaving a nice shell textured pattern. Queen scallop shells were just the right size for this job.
The top middle arch bricks were made more tapered to ensure they plugged the gap between the two sides and would not fall out. The two rear bricks were turned flat, so that the 6 round perforations can act as a chimney, with another pair of bricks on top of them, which can be closed off with pieces of old kiln shelf.

The moment of truth! The tapered supports were knocked away and the former dropped. The arch bricks sagged slightly and were gently tapped into being aligned in a smooth curve.
This images shows the gap about the former, the horizontal supports and the tapered legs.

The gable ends were built up with old kiln shelves and bricks. The front door is a bit scarred by a firing in which an experimental copper casting went a bit wrong and covered the shelf with burnt in copper (luckily, by good management, it was at the bottom of the kiln!).
A pair of holes drilled through the door enable a looped metal tube to act as a handle.
The back gable is kiln shelves facing the inside of the oven, with bricks and blocks holding them in  place.
The front door is flanked by bricks. To stabilise the gables 2 pieces of threaded metal bar have been braced between them, the nuts tightened up to hold them in place against metal kitchen drawer runners poked down the holes.
A larger bar is placed across the front to hold the top of the door against the arch. 

A closer oblique view of the oven.

A couple of side panels from an old electric kiln are placed to form a pitched roof over the arch.
The next layer over the brick arch is clay, with added sharp sand and wood ash. The clay is reclaimed earthenware pottery clay, some is commercially prepared and some is dug locally.
The clay was mixed by shoveling the ingredients between large buckets. The water content can be adjusted to give a mortar like slumpable consistency. Before use some white Portland cement and hydrated lime will be added, to turn it into a hard shell, which will be stronger and more weather resistant. This will be poked down between the gaps in the arch bricks, to hold them in position and partially fill up the 3 holes in those bricks.

So that is the first layer applied, as described above. The moveable bits - the door and chimney dampers have been wrapped in plastic to stop them being cemented in place.

A ceramic sheath has been inserted in the roof (where there was a small hole which was drilled out a bit more), that will be used for the temperature thermocouple, connected to a digital read-out.

Next day, the mud pie covering has dried and cured a bit. And it has cracked up slightly as it shrunk against the bricks underneath.

A thin layer of the same clay mix was applied all over the oven, then a piece of very open weave sacking was laid into that and squished down.

Lengths of plastering scrim, were incorporated over that layer, to give more green strength.

Bill then applied another layer of clay mix over those layers of scrim and trowelled it firm and flat.

When that has dried and cured a further layer was added, using the same mix, with more lime and cement and with added insulating material - vermiculite granules.   That was trowelled on fairly thick and the corners rounded off.

The other side!

Front view

At Last! A use for all the ceramic shells that are generated in the wood fuelled kiln - clay filled sea shells are used to support the pots to stop them sticking to the shelves when the ash reacts with the clay. Each firing produces hundreds - of difference sizes!

Plan A was to form a shingle like roof by impressing the shells into the last clay mix coat, which would have helped it to be more water proof - not just the lime and cement mortar.

However, the shells would not stick into the clay mix very securely, so they were embedded flat onto the clay mix. So more decorative and less functional. But perhaps we should have tried harder to clean the shells - the actual shell turns to quicklime in the firing and the shells were just dumped in buckets, so the lime rather stuck to them and had to be wire brushed off. Also the clay mix tended to get onto the shell surfaces. Perhaps they will need to be grouted and then cleaned again?

Front view.

Other side view.
The whole thing was left to dry out and the mortar cure.  A gentle pre-heating and then cooking!

The catering division preparing pizzas.

The hot oven, with a bit of a failed pizza heating up.

Not enough time was allowed for the initial heating. So the radiant heat from the ceiling arch was great and cooked the toppings very well, but the base bricks were not really quite hot enough and cooled down to below optimum temperature. However the pizzas were declared rather splendid.

All over and cooling down.

During the initial heating a short piece of stove pipe was used as a chimney, which helped to create an updraught and get the fire burning well. During the cooking a small fire was kept going down each side of the chamber.