Supports simple and quick filament change / color change
You can easily and quickly change the filament / color with a second spool holder (I used this one). Meanwhile, you can prepare for the next filament change without having to rush. Additionally i recommend the “Elegoo Neptune Filament Clip - Color Change Buddy”. Edit: V2 got these!

Use M600 gcode command or pause print command at needed layer.

Better guidance and tension
With this holder, you move the filament sensor to the center below the spool holder and thus improve the guidance and tension of the filament.

Compatible with all 2020 V-Slot extrusion rails
The holder is specially designed for the following Elegoo models:
Neptune 3 Plus / Max
Neptune 4 Pro / Plus / Max

Neptune 3 Pro and 4 (standard) got its own version as these models do not have 2020 extrusion!
This is attached with the two screws of the original spool arm. Longer screws should not be required!

Print
The model is designed to be very accurate / tight. If you have not calibrated your printer, it is better to print the model with +- 1-2% scale.
Print up side down and use supports.

Installation
1) Unscew the filament sensor and the brace and keep all scews.
2) Simply place the holder over the spool holder and press from above - it snaps into place audibly. 
3) Install the filament sensor - No additional screw necessary, simply use the existing one from the filament sensor.

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Differences of the remix compared to the original

• Mount for the 2020 extrusion enhanced in stability
• simplified the whole design to make it more stable
• rounded some edges
• made it possible to attach the filament sensor after installation of the mount
• added filament clips in V2