To give users the impression that the mail server is running on their own domain, the owndomain.com/mail works great.
We also use the owncloud functionality, especially the calendar function.

Is it possible to have the /cloud location also available, like /mail on all domains?

I get what you mean. To further simplify.

box.domain.com/mail and domain.com/mail is working.
box.domain.com/cloud is working but domain.com/cloud isn’t working.

See https://github.com/mail-in-a-box/mailinabox/issues/278.