The guide you link to shows a Tiptop µZeus power supply being used to power the cardboard Eurorack case, with the Tiptop µZeus being mounted directly to the side of the cardboard case.
I do own a µZeus and know how very hot its innards can get, which the µZeus manual also mentions: “WARNING: The internal components of the µZEUS can get very hot. Do not touch the internal components while the µZEUS is connected and/or powered.”
And some paragraphs down it says: “Do not install the µZeus directly to wood or any other flammable material, this will cause the uZeus to overheat.”
While I have not yet heard (luckily!) of any overheated µZeus due to poor installation, let alone a fire accident being caused by a µZeus mounted in a cardboard box, I’d rather have my µZeus living (and breathing, for that matter) in an open frame made with metal sidecheeks to which the rails are properly mounted.
Please consider: Even the cardboard box needs several comparatively expensive components (compared to cardboard, that is) – the µZeus with flying bus boards at 85 USD, the rails at 40 USD, and the power adaptor at 25 USD, totalling up to 150 USD. For just 17% more you get these metal sidecheeks cost at 25 USD, or way less if you have a metal workshop somewhere in reach.
Imagine the worst happens and such a cardboard box catches fire, sets the building aflame – and then the police and insurance company do a thorough on-site investigation and find out that a “misfit electrical installation” caused the fire. That’ll be a lot more bucks to shell out than 25 USD for an installation according to the manufacturer.
Disclaimer: I am not an electrical engineer nor a firefighter, leave alone an insurance agent, and in no way affiliated with Tiptop, I am just risk-averse (and old!
).