Super hot J plans: have you used them?

I am preparing to build Mat Walker's core this summer. I shared the plans with a friend as I am struggling to find the Ceramic Fiber board at what seems like a reasonable price and in the size Mat uses in his plans (I can get smaller pieces online sufficient to build the core for about $250). In looking at the plans my friend became concerned that the bricks don't fit inside the assembled CFB, and noted the inside dimensions are not 6x6 but something smaller.

Here are my friend's notes:

"I drew all the pieces in SketchUp.  I've taken those and tried to fit them together how they  will be in the stove.I ran into a few problems.  One is that the fire brick assembly does not quite fit in the ceramic fiber board (CFB) assembly.   The plans don't call out the size of the fire bricks.  I've been using what seems to be the standard size (9x4.5x1.25), but maybe that is not right.  It's not that far off, just an 1/8" or so in two places.  But as expensive as this CFB is, I'd want to be really clear how things will fit together before it gets cut up.

Other interesting things about the plan.  A couple of the pieces (inner sides) are dimensioned a few hundredths of an inch off of whole inch sizes.  I think this is likely just an error on their part.  I think it would be quite difficult to cut things accurately to a hundredth of an inch, and I don't see the need for it.  Another thing is that the plans are titled "SuperHot J Six Inch"  but I am measuring the riser as 5x5 inches inside and 7x7 on the outside.  IDK if that is an error or what to do about that.  

I'm confused about the riser.  In the video he talks about tubular risers.  In the plans he shows a riser assembly made of 24x6 CFB.  Which do we want to use and how long does it need to be? "

I am heavily presuming the plans do work: I haven't heard otherwise here on permies, but i'd love specific feedback regarding Dan's concerns.  Thank you all very much

Hi Barbara;
Have you emailed Matt with these questions?

I have built Matt's riserless core and his plans are not meant to be exact, they are a  suggested pattern.
Every build has different dimensions.

I assume the firebrick your J-Tube needs is the feed tube.
Split bricks of 1.25" are used.
5.5" 5.5" is the published ID on a 6" J
I suggest building it at 5.5", as you want every little bit of room in the feed tube.

As far as risers,  Matt's stoves originally used flat CFB board in a square.
I believe that he is now leaning toward using a five-minute riser constructed with Morgan Super Wool.