I wanted a longer and wider sidebox for my MK3S+, so I took dotson90's excellent Mesh Sidebox and remixed it, albeit with only the left sidebox. However, you should be able to mirror this model to create a right sidebox as well.
I made the mounts wider to be able to clear the rails caps and it's not so long that it blocks the SD card slot (see pictures above). However, you can make it longer and wider to suit your needs.
The .3mf file already contains the layer color changes. However, it is NOT configured for a MMU2S like the original so you'll have to manually change your filament when the printer pauses.
The uploaded gCode files use Prusament PLA and Prusament PETG filament settings for those that want to print right away. I used PLA for mine. The gcode files also contain the layer color changes.
NOTE: You'll need support for the mounts on the side of the box!
The author remixed this model.