Johns Manville Thermo-1200 Calcium Silicate Insulation 2" x 12"x 36" blocks (qty 6, plus 2 blocks I had cut from) These are rectangular blocks (image on the box shows insulation for pipes for some reason).

I cut two of the included blocks in this case, but you can have the excess from those pieces. From the manufacturer:
"Thermo-1200 is a water resistant, Type I calcium silicate pipe and block insulation, designed for applications that operate at temperatures up to 1200F (650C)."

Here is the datasheet:

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