Hi Phillippe,
I think I answer most of your questions about the insulating mix in this post (oldie but I’m fond of it):
Specifically: the BWO design includes a “void” in the slab that is designed to hold in the insulating mix. It is the same length and width as the firebrick hearth that will be placed above it.
You’ll see from the post that it is a relatively wet, sloppy mix and it needs to cure in a form, just as the slabs themselves do. That’s the purpose of the void. When it is cured, it will support the weight of the firebrick hearth without crumbling. The arch rests on the outer edge of your slab, so the insulating layer does not bear much of its weight.
It will come out like ordinary concrete, because you’ve made an aggregate of Portland cement and volcanic rock. It won’t pick apart like low density foam.
It is extremely easy to mix. Both materials are lightweight, and they will mix easily with a hoe. As you’ll see from the linked post, I mixed the two in a wheelbarrow, adding appropriate amounts of water. I used a materials shovel to lift the material on the slab and into the void. It really is easy and relaxing compared to most of the tasks involved with building an oven.
For your build, where I see you have your slab poured from the second post you made, you will want to build a “curb” on top of that pour. Use an outer form as you did for the main slab, and an inner form to the dimensions called for in the BWO design. You’ll need to extrapolate a bit from the three-slab design, but all the information you need is there. Remove the curb forms when your concrete is cured, then pour the insulation layer inside it.
Drain holes are not necessary in the slab, especially if you’re going to build the galvanized lean-to structure over it.
I don’t recommend the dry-mix method you suggested. You will find the insulating mix is not nearly as heavy as concrete mix, because the Perlite weighs practically nothing and the Portland cement is not mixed with stones and pebbles (aggregate)—aggregate is over half the weight of a batch of concrete. If you want to use your electric mixer that will work too.
Hoping all this helps you put this part of the job into perspective Phillippe. (And I’m glad you have good dogs guarding your build!)