Patch cable organizer. You could use it for your breadboard prototype or any other short cables.
Gap between rails is 1.8mm in drawing, it could be different after you print (under/over-extrusion, steps calibration etc.)
Generally if your cable tip is larger than 1.85 I expect it should stay in place and not fall off.
I printed this with eSUN PLA and it's flexible and strong enough. Printing orientation is important. You will need supports which can be broken easier. Adjust support Z distance accordingly. I broke most of it with my fingers but the last parts I had to remove with exacto knife, took couple of minutes.