If you don't want to, don't know how or cant use quota stuff - You can create image files on your main storage for each cam.
64GB container ext3 or fat32, choose one.
dd if=/dev/zero of=cam03.img bs=1M count=64000 # mkfs ext3 -F cam03.img # mkfs fat32 -F cam03.img
Create a mount directory
mkdir /mnt/cam03 chown nobody:nogroup /mnt/cam03
Mount the image
mount -o loop,rw,sync /mnt/MAIN/CCTV/cam03.img /mnt/cam03
You still have to deal with remounting the image on each boot and making sure NFS remounts too.
My MAIN are 4 2TB disks using ZFS, so I have good redundancy and fast read write. This worked pretty well for me.