I designed a stackable spool holder with optional swatch holders as well as an optional levelling base via base bolts. 
It is designed to hold any major makers 1kg spool of filament and can be stacked as many times as you would like it to be. 
The holder has an angled base that shifts the weight of the filament spools to the base supports.

Save space with a vertical spool holder instead of a horizontal one.
 

A convenient swatch holder to easily see what filament is in the rack at any time (designed for 24mm x 24mm x 2.5mm and provided spacers are required for swatch holder use.)

A levelling base with with a slotted 1cm m20 bolt pattern to level the stand on uneven surfaces!

Stack as many as you would like!

The tolerances on the spacers and swatch holder are forgiving taking into consideration different printer tolerances.

Super Glue is recommended for final assembly.