I'm also looking to build a DIY Air Cleaner in Europe, and struggling with some of the parts. Have there been any new findings in terms of parts / materials, that better support this project?
In particular, one of the most difficult seems to be the box fan. The Pearl model you linked now costs 80 EUR (https://www.pearl.de/a-NX6414-3033.shtml), which points me to looking at alternatives.
I've come across some others like these (https://www.led-grower.eu/ventilator-boxfan-one4air-o30cm/, https://www.growland.biz/Box-Fan-30cm), but I'm not sure if those designs and fan styles would work.
Another option is a design like this (https://twitter.com/CRBoxKits/status/1598165384267386880), which uses smaller fans that are more easy to acquire on amazon.de etc. Though I think I'd prefer to try the classic CR box design, for ease of use, portability, and replicability. Just need to find the fans!