How to keep objects in same zone when they collide?

I made a zone, cylinder with zero height. When one sphere moves to that zone it is fine but if another one moves to same zone will show just one sphere. I tried to add rigid body for each sphere but they collide inside the cylinder and both fly. I want both of them to collide but stay inside the cylinder zone with separate distance between them like 2 units.
Please help

you can use a cylinder mesh with reversed normals, things will bounce inside it.

you can add a new force to the spheres, that bounces them back from the cylinder like rubber, by using a isosurface cylinder formula from the net, and make the new force act on the spheres if they go outside the cilinder isosurface value, and send them back in straight away.