Looks good! I'm not sure that you need Nema23 motors for that size printer, but if you do use them, pick ones with a rated current of about 2.5 to 2.8A so that the Duet can drive them directly.
Go with fire rated drywall. I'd maybe even look into a self releasing fire extinguisher.
maybe the paneling they put behind and around stoves? might be cheap, should be fire resistant. the main issue with fire is the spool igniting, maybe make that in a fireproof box and the rest fairly fire resistant. if you can keep the spool igniting it is a factor of print size. if the spool ignites its a much larger fuel source. I am not sure how much you want to worry about it, but then having a system that removes later worry during long prints is awesome goal. I think a narrow metal tube will cutoff the oxygen and quench a burning strand of filament, needs testing but it seems resonable that a section of 3mm ID brass tube would stop fire advancing along filament. bent and polished it would be low friction, it would lead into a fireproof box for the filament.
No chance to use a longer screw so it's supported from both sides?
I had the endstop brackets and 4 of these 2020 rail alignment peices sls printed.
[attachment 100404 IMG_5427.jpg]
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This is the best way to mount 9mm that I could come up with. I tried a few methods and even had toothed. This way had less belt wobble for me.
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