A hole on each side of the top marble trays would be helpful. Marbles could be dropped into the tray through the holes. Then, clear plastic could be placed over the front of the trays and the marbles would be more stable.
Great idea! I’m always finding a spare ball on the floor.
I purchased small containers from the dollar store for the marbles. They fit nicely in the larger open area. As they work, I have the students place the containers inside the box lid so if they spill, all they do is roll around the box lid. I think I got 10 containers for $1, enough for 5 Turing Tumbles.