/etc/fstab:
----------
/dev/mapper/crypt-tmp           /tmp                    ext2            noatime                                                         0 1

/etc/conf.d/cryptfs:
-------------------
## /tmp
mount=crypt-tmp
options='-c aes -s 256 -h sha256 -d /dev/urandom'
source='/dev/hda9'
pre_mount='mke2fs ${dev}'
post_mount='chown root:root ${mount_point}; chmod 1777 ${mount_point}'

Error at boot:
-------------
Skipping invalid line in /etc/conf.d/cryptfs: mount=crypt-tmp
...
mke2fs 1.39 (29/05/2006)
fsck.ext2: No such file or directory while trying to open /dev/mapper/crypt-tmp
/dev/mapper/crypt-tmp: superblock could not be read
...
(try with command)
e2fsck -b 8193 <device>