Originally based the filament holder using the FlashForge spools of filament, since up until now, I had no way of storing different types of filament. Although simple, the design takes into account outside and inside length of the rails, diameter, and tolerance.  

For those interested in dimension references they are as follows:

• Rail Thickness (CAD) - 0.15 in. (~0.145 in. once printed) + (0.015 in. clearance)

• Inside Rail-to-Rail spacing - 2.470 in.

• Outside Rail-to-Rail spacing - 2.770 in. 

• Maximum Diameter of Rails - 7.875 in.

The part itself 5.75"x3"x1.5" LWH, and does shrink when printed using PETG. The slicer software is setup with FlashForge PETG Pro in mind, but can easily be swapped for something else.

The front side does have a small angled portion which has been created incase you want to label your filament. I use these I found on Amazon: Avery Multi-Use Permanent Labels, 1" x 2.75", White, Non-Printable, 128 Blank Labels Total (6113). It's much easier to write the name of the filament before applying, even though the surface is surprisingly smooth. 

And also I know the image I have displayed is pretty low on the filament, but when I was designing the part, I used an unused filament to get all the measurements, so even spools that haven't been used, will still easily rotate inside the part.