I was inspired by https://www.printables.com/model/418630-bambu-lab-integrated-hepa-carbon-aux-filter but this is a redesign from scratch. My 10x3 magnets were in reality only 2.6 mm thick so this cage is made to fit four of them stacked - in four places. That bundled with some ridges make the cage sit very tight and secure.

Find "GL2103 HEPA carbon filter" on eg. Amazon. Remove the plastic cage it comes in, and put the bare filter in this cage. Remove the three screws on your aux fan housing, put corresponding magnets on the inside of it with a drop of super glue (guided by the cage's magnets on the outside). I think I used 16 magnets on the inside as well, they barely fit between the fan and the housing. After that trivial installation, the cage is easily removed for filter replacement with no tools.