Case read access

Currently, I have 6 BU.

I would like to allow 5 BU to read cases from one another, & 1 BU will not be able to read from these 5 BU and these 5 BU will not be able to read cases from this 1 BU as well.

I do not want to create a Parent-Child relationship to handle this situation

Any way would I be able to achieve this? Kindly advise.

Thanks.