Hi Iggy,
The first part of the instructions for the Insulated Base (steps 1-18) are all about how to build the slab with a space for the insulating layer. Steps 46-50 describe how to mix and pour the insulation into that space once you’ve placed the slabs on top. That section (pages 15 and 16 in the instructions) are what you’d want to look at.
Since you don’t have that space, you could certainly choose to use the silicate insulation board instead of mixing the Portland cement/vermiculite. The advantage of the mix is that it is both very effective and inexpensive. The disadvantage is that it takes a day to dry (I recommend you don’t use Perlite, as it takes several days to cure vs. Vermiculite which sheds water and cures quickly).
Either way, you still need to build the oven slab up to contain both the insulation and your firebrick hearth. Look again at the link above for general insight, and this particular post has the dimensions for the insulation cavity:
I hope this gives you enough guidance to move forward, but keep hitting us up here if it does not. We want you to succeed!