OK, I've learnt some lessons over the last couple of weeks.
Firstly, I've realised that in making the oven symmetrical I'm losing too much heat out the door. I can see the reason now that a lot of the ovens pictured on the net appear to be long a la Quebec style. I'd strongly suggest you do the same. The area of even heating I get in my oven is towards the back. So, if you want to fit more in, just extend the back further out.
Secondly, I'd make the walls much thicker. In my case the walls are about 10cm. And yet they are still too hot to touch and they are visibly losing a lot of heat (in the form of steam, convection and radiation - you can see the steam and the heat shimmers). Why lose heat when I could have it better insulated. So make a really thick insulating layer using clay, pumice, vermiculite, straw, whatever.
Finally make the door big enough at the bottom to fit your pizza peel/implements. It's painful squeezing food through a narrow opening.
All that considered - it still has performed far better than expected. I'm really quite impressed at how good an oven it is, and also at how efficient it can be.
Go build one yourself!