There are hundreds of spool holder designs on Thingiverse, but this one is designed to match the cool look of the FT-5 enclosure in sleek, solid acrylic. Although it was designed for the FT-5, it should work on any printer with exposed 20x20 that needs a side-mounted spool h order.
If you print in a strong filament like ABS, this holder will support up to a 5 Kg filament spool. Its designed to fit snug against the enclosure walls anywhere you can connect to 20x20 extruded beams.
Note that it requires a length of 1.5" solid acrylic or wood rods. You can print it yourself here, or if you have trouble finding the acrylic (or don't have the equipment to cut it right) this FT-5 spool holder is also for sale on my website with all of the parts ready to go.