This won't interest everyone [perhaps even no one], but I thought there might be something here for some of the DIY crowd.
Weather permitting I like windows open, which means dust. We've got old air ducts, which means dust. I don't mind cleaning out PCs, but there are many other ways I'd rather spend my time, so I use filters to, as possible, keep dust from getting inside the PCs. I've been using filters long enough that when I look for a new case, how easy it'll be to filter is one of my main concerns. What I use for the filters themselves is pre-filter material for an older air purifier/filter -- it lasts a long time [I've been reusing what I've had for years, just now ordering new], washes out easily enough, & even dirty doesn't seem to much restrict air flow.
I have 2 of each filter so I can swap them out, quickly blow out the case if necessary, then wash the dirty filters for next time. The easiest way I've found to attach rectangular filters I've cut out of this material is to use the the coarse 1/2 of stick-on Velcro at the 4 corners, but the corners of the filters started to get a bit ragged after a couple few years. I switched to magnetic tape, which works well, except the self-stick adhesive tends to come loose when I wash the filters. Nonetheless I made do until I tackled another cooling problem, which forced me to find something I think will work better in the long term.
I run a larger self-contained water cooling setup for the CPU, with the radiator mounted at the top inside of the case. If the fans blow up & out, in several months, despite the filtering on the case, the radiator starts to get pretty clogged -- that means pulling the cooler & dipping, washing & rinsing the radiator, letting it dry, & putting everything back together -- NOT FUN. So the 1st time I did this complete cleaning job I reversed the fans to blow inwards -- that lets me stick a filter on the top of the case where the air enters. Works great for dust, & the radiator's stayed reasonably clean, but now I've got too much air pressure inside the case so air flow & cooling suffer. The dilemma was how to have the fans blow up & out through the radiator, & still filter the air those fans are blowing through the radiator. The product I found & got working is Very imperfect, but it was the closest I could find to a relatively easy solution.
What I bought was a set of (5) plastic fan grills with covers to hold filters. There are holes in each corner, designed so that the same screws that mount the fan hold the grill in place -- the plastic's far too thin, so just drill those out if/as necessary. I used a hot glue gun to attach the grills upside down, but most any good adhesive should work to attach the grills to the fans. Be careful as these grills are slightly larger than the fans -- since the 2 fans in my case butt against one another, I had to made sure they were flush with one side of the fans.
Next, the covers are made to sort of snap onto the grills in a way that I couldn't imagine working well. The reason I attached the grills upside down is they have a slight lip I wanted pointing outwards, so I could glue 4 small washers next to the screw holes, again using the glue gun -- I used the slightly thicker steel washers for pop rivets. Then I set a small magnet on each of the washers, applied a dab of glue to the top of each one, then set the cover down on top -- it seemed the simplest way to align all 4 magnets & glue them to the cover. It seems to work OK, holding the covers in place upside down, yet they're easily removable to replace the filter material I cut & fit inside -- I didn't like the included sponge filters at all, especially since they stunk pretty badly.
Now since I had 3 grills & filter covers left, I threw the grills in a PC parts box, then glued magnets on the inside of the covers -- they're perfect now to hold filters in place on the outside of a PC case without the hassles from Velcro or magnetic tape.
The filters I bought from Amazon are here, but it's something fairly common you should be able to find in the EU without much trouble.
amazon[.]com/Aftermarket-Carbon-Filters-HAPF60-Filter/dp/B00L2NZOKK/184-2306097-1195459?ie=UTF8&ref_=cm_cr_arp_d_product_top
The grills & covers were also bought from Amazon, but I've seen listings for the very same thing, and in additional sizes, on Ebay.
amazon[.]com/120mm-Computer-Dustproof-Cooler-Filter/dp/B010ESDROS?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o05_s00
ebay[.]com/itm/Dustproof-120mm-Case-Fan-Dust-Filter-Guard-Grill-Protector-Cover-PC-Computer-BLK-/281483943753?hash=item4189c01f49:g:z8IAAOSwdpxUVBW6