I made a case to protect my darts when taking them somewhere. Here are some more images. The case was made to fit a specific model and it's not very parametric till then end, so I'm afraid it's not super useful for others.
It was made in #FreeCAD 1.0 which was super smooth!
They fall out. For now I use a rubber band around the case but I don't like that solution too much. The initial idea was to put a rubber band around the "waist" so that they would be held there. Unfortunately the rubber then doesn't touch the dart. My current idea for a fix is to make a clip that pushes against that darts sideways through the holes.
If you wanted to try to make your original rubber band idea work, you could try moving the central dart up slightly so that the rubber band forms a triangle shape and adjust the contact points between the rubber band and the darts to be at each corner of that triangle