This is a mount for the Filler Filament Holder on the IKEA KALLAX shelf. It can be mounted to the corners using the big screw from the shelf. I have printed it from PLA and used it without issue for more than a year.

I recommend adding supports for the overhang.

Print the main holder part from:

https://www.printables.com/model/174620-filler-filament-holder