Basic design to help kids make 180 degree turns with dominos. Design is a bit "loose" fitting. You might have to scale it larger or smaller to fit your dominoes. Width of each slot at circle is 8mm. Depending on expertise and dexterity, might want to also consider making it taller to better hold dominos. Height is 10mm.
You would obviously just put dominos in the slots around half of the circle, and then pick up piece leaving dominos behind.