I've seen these DIY air purifiers before and thought a out making one but am unsure how well they actually work. Has anybody here actually made one before?
I've lost track of it, but there was a study published about this style of filter around the start of the pandemic that suggested it could be effective at reducing PM2.5 and possibly airborne virii. It's a sound idea conceptually -- continuously filtering and re-filtering room air at a high volume is how industrial clean rooms work, and while this is a far cry from a carefully-engineered system like that (you won't have the hygienic surfaces and laminar flow characteristics, for instance), it at least has more than enough airflow to filter a room effectively.
Effectiveness is pretty much entirely dictated by what filter you choose, assuming you don't mess up building it and leave huge gaps. This guy on youtube made a video showing the effectiveness of different filters: https://www.youtube.com/watch?v=XAJ__E-hIJc
I have one with just some cheapo filters in my workshop. Does a great job of keeping dust out of the air for paint and finish work.