This picture is out of sequence, but should give an idea about the layers of insulation under the oven.
Starting at the bottom, there's a concrete heart.
On top of that I poured a layer of egaline which is just to make the hearth level: it's basically a concrete that you pour when it's as liquid as a milkshake than slowly becomes rock hard and completely level.
On top of that there's 6 cm of Ytong blocks, which is a lightweight insulating brick. (at first I wasn't planning on these but they're dirt cheap, and the cooking floor could use some more height as I have a bad back)
On top of that, a complete layer of aluminum silicate board I had laying around.
Then there's another layer of the same stuff, but with a 105 cm (42 inch) hole in the middle. In other words, it's only under the soldiers, not under the cooking floor.
The reason behind that is: I'm not sure about the cooking floor bricks. I've read a lot about bricks, conductivity and whatnot, only to realise it's all a bit too complicated for me and I'll get there through trial and error. Hence, the whole floor can be dismantled through the oven opening and another floor put in place.