Most info says to lay a concrete slab, build a brick base and then lay another concrete slab ontop to then build your oven ontop. this is consistant but has variations.
the top slab has many variations you just need to pick the one right for you, some/most lay pavers or firebricks ontop of the concrete slab but my research suggests that laying pavers directly onto concrete won't give you as much bottom insulation as you need. If you use fire bricks as your base then you should be ok.
What I am doing is laying the top slab and then adding a layer of Hebel on top and then clay pavers, the Hebel is the same height as a brick so it will all even out....i hope.
Hebel is apparently a good insulator, the website official says and so do many posts.
note: you will need to use the proper hebel glue/cement to join the Hebel, but guess what ? the guy who gave me the Hebel also gave me the glue ! ....you just gotta ask and you will receive.
"Reuse BEFORE recycle" , this is a much better eco way for this world, we disgard so much and I have found when I have said this to people they have responded positively.
The better designs I have seen all have an insulating bottom layer.
Variations I have seen that I would consider ok are "childrens play sand" and clay, investigate natural clay oven bases on youtube as there is some interesting stuff on that, eg: clay with wine bottles and crushed bricks as the base to the oven.
There seems to be a consistant view that there needs to be a bottom insulation layer.
