I've been using some crap 120mm filters that I stuck together with duct tape for my Corsair H100 240mm radiator before I realized that I have a 3D printer big enough for such a print.
The design should fit onto any 240mm radiator and is optimized to use 12mm carbon filter foam.
Included is a file for a standard 120mm fan filter (as well as the filter template) which I haven't actually tested, however the STP files for both are included so you can do whatever you want with them.
All comments, suggestions and tips are greatly appreciated.
Printed in Black PETG, at 50mm/s
Cutting out the Carbon Filter
I used a 12mm thick carbon filter that I got from ebay and cut it down to correct size.
The PDF template files for the carbon filter are included in the thing.