I've designed my own version of a spool holder that is stackable, turning it into a rack/shelf system. 

I designed it to be as economical as possible. There are many holes & cutouts to reduce weight & filament usage. These holes also can be used to stick the filament through to keep it in place, and you won't have to cutoff the bent end because it won't be bent with this design!

You can stack these things pretty high without them falling over. I recommend putting rubber feet on the bottom holder to prevent further movement. I currently have 10 stacked ontop of eachother.

Series available:

• V2.0.1: Standard height, fits Bambu spools & smaller spools.
• V2.1: Slightly taller height, fits slightly larger spools. Stacks the same way on V2.0 series

Key notes & features:

• Fits a wide variety of spools from different brands
• Holes for filament end support, no need for a clip.
• Designed for minimum filament usage
• Stackable
• Sturdy, strong design. Weight/gravity forces go into the walls of the holders below.
• Circular design, stronger than a regular box design.
• Glue them together for maximum stability! Not required though!

Changelog:

• V2.0.1: Added steps to the top stack area for easier removal/entrance of a spool. Changed design at the floor entrance area for a better quality print.
• V2.1:  Created as a taller version of the V2.0. Currently uses less filament than the V2.0. Changes are being looked at for the V2.0 to use even less filament. Recommended to print instead of the standard V2.0 if you want to save 10g of material.