That foam might be available through music retailers for its acoustic properties? Someone in China will make it cheap
One trick I can think of with the firegib is that, in normal building, it has to be screwed to the wall as glueing it won't work cos fire melts glue, gib drops off, fail.
The only thing I thought might work would be to glue in the four sides then the end piece as a "spacer" pushing them all back against the outside.
Good luck with your build
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