I have made an adapter for IKEA closet to mount a rod for filament spools.

I am using a broom stick which is 22.3mm thick,

But basically any rod which is up to 22.5mm thick and can be cut to length could work for this project.

I made 2 versions of this :
1. Is made of a 80mm offset (total length is 106mm) and can be used with 1 front rod and laying the spools on the shelf.

2. Is made of a 50mm offset (total length is 76mm) and one without and offset which should be used with 2 rods (front and back) and is not laying on the shelf.

for me I used both option to be able to mount 2 rows of spools in the same shelf.