Since I didn't find one that worked for me, I made my own.
These caps will fit a Mobius camera with standard lens. The filter cap is open at the front and allows you to put a colored plastic sheet or so-called filter gels (i.e. ND filter) into it. Cut it into a ~18mm circle.
Excuse the horrible example image, I only found one kind of colored plastic foil (my filter gels are still in the mail) and it turned out to be pretty ugly.
Prints without support. I used PLA, ABS will probably be a bit easier to clip on as it is more flexible.