I had a number of smaller filament spools that would slip through the rack on my modular filament racks.

Original: https://makerworld.com/en/models/97332#profileId-103873

Less supports: https://makerworld.com/en/models/152626#profileId-166657

I had designed a different base piece that had the rails closer together but wanted to create a more customizable solution to the problem.

These clip on components can be attached to your existing rails at any location for any length of rod you chose. If you need full length use full length. Only need space for a handful of small spools, make it short.  

If you need it to sit lower or have slightly larger small spools you can increase the length of the center arms which will spread the rods further apart but will also cause them to sit a bit lower.

Designed for the ¾ or 19.3mm dowel rod but all parts can be simply sized up in your slicer to fit the dowel size you have.