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The box is very nice (think smartphone box), almost too nice for the "carbon monoxid detector" aim of an earlier discussion.

Build pictures. White cardboard structure, Anaglypta relief wallpaper, black paint. The box can also be used standing on a small side. 

Starkvind fit tightly in the large side of the cheap triangular Vesken shelves (<10€). Block the other sides with fans in cardboard using tape: you have a filter shelf.
A short description of the process. You can find a few pictures here, but I had some trial and error process and even with the final ones, no warranty it will hold long, as @nickelpin also explained very well ! https://twitter.com/Trematomus/status/1560996719222853633?s=20&t=46yN6fEVlw45KlibojHaIA