The easiest way is to use split approach.

But it will work only if you add a username inside the directory you want to share:

Please note that first you create empty ?sharedir? directory, and read more

In rare cases customer wants to move /opt/alt to different place. To do this:

Move /opt/alt somewhere (mv /opt/alt /var/opt_alt).
Create /opt/alt directory (mkdir /opt/alt).
Bind mount new to old: (mount -o bind /var/opt_alt /opt/alt).
read more