Lazy man's self-solving marble maze. Warning: I know NASA have a 3D printer in space, but this will not work on ISS only on Earth. It needs gravity :-)

See how it works (YT): https://youtube.com/shorts/2j58FhQyd1Q

Or here (FB): https://www.facebook.com/watch/?v=468390011863196

3D printing without supports. It can be printed with 0.2mm layer height too, but if you have time, for smoother small angle sloping surfaces, I suggest using a smaller layers (0.15 or similar). But with 0.2mm it works too with 90% success (or just shake it a little bit if ball stops). Designed sloping angle is 12% (1.2mm down on 10mm length). You can play with scale in Z direction if necessary. Version presented in the video is printed with 90% Z scale. 

Another suggestion, print it in some light color filament, then you can color the top edges with marker. 

Both versions are available for the download (regular and self-solving mazes). Tested with 6mm ball but 8mm will work too.