Wood box over horizontal flue tile

I’m looking for ideas for a wood box, specifically constructing the bottom over a horizontal section of flue/bell. Originally I had anticipated using some sort of heat tolerant cement board that would span from the front to the back of the face, and then pouring a reinforced slab of sorts. I spoke with my local fireplace store and I was advised to not permanently incase the horizontal flue tile, as it would possibly fail in it’s unintended orientation. I am leaning towards my original idea, because I think it needs that uniform support and a solid base to continue to build vertically upon. I have included some pictures, the stone is not set in mortar, just playing around with it. I can completely fill the void on the back side of the bell, that should help minimize the “bridge” across the flue tiles. Now, I did read about building directly over these horizontal flue tiles in the case of a heated bench, and I am supposing I would need to completely fill the area under the flue tiles with brick as well, as it is currently only supported under the seams.

Concrete mix ratio advice would be appreciated, it seems it would need to be somewhat “refractory” in nature.

If building directly on the face and flue is acceptable, (perhaps with some fibrefrax on top?) I have flagstone and red clay brick to work with, but I am hesitant to do this as likely somebody will less than gently toss a log in there.

While I am at it, I want to basically create an identical “slab” over the wood box to place a water heating tank, as I have a heat coil installed. Also there is the capping of the heater core looming out there…

The rest of the heater face is to be done in clay bricks, and I am supposing that due to the weight of the water above the wood box it should be double layered. Any thoughts about supporting the middle “wall” of bricks that would be on the wood box side of the bell? Would rebar in the poured slab be sufficient, or perhaps angle iron along that edge of the slab?

Please excuse my possible incorrect use of terms, I am sure there is a way to accomplish this, just can’t quite wrap my head around it.

I decided to just go for it with the flagstone and a cardboard expansion joint. I also put angle iron to support the edge next to the vertical flue tiles. I’ll build the wood box on top of this.

Wow Ruth! Congrats on your progress! It’s looking great!

Leaving the flue tiles room to move and using an angle iron to support the wall above it are good precautions.

By wood box, do you mean something like this?

Thanks Max! Yes, wood box similar to that, it’s not gonna be quite so perfect though, that is beautiful!