Did anyone ever visit the film's website? Possible answers.
It's amazing, I'm not going to lie to you, it will almost be 20 years since the film has been released and a solid 20 years of searching for answers among fans. I've been looking for the answer of what is the cube for almost 12 years, on and off of course! I started when I was 13! Either way, after countless discussions about that ending in which Vincenzo Natali destroyed it and the concept of the alien fruit which grew on the walls of the cube, we don't have much information of what is the cube when it comes down to the one on the film. However, I once browsed the films website which was released in 1998 and the website was this https://web.archive.org/web/19991012072133/http://cubethemovie.com/CBSeal.html
Based on the website, it gives a vibe that kinda describes the cube as a government experiment which fit more into David's statements.
Check it out!