@grimsdottir swap can potentially contain cryptographic secrets. /tmp is more that it might leak your system data in general. Fortunately both are easy to setup and don't need further interaction upon boot etc.
To setup the swap you just need an entry in /etc/crypttab + /etc/fstab and cryptsetup will do things automagically for you in 99% of distros.

/etc/crypttab:
cswap1 UUID=1234...cdef /dev/urandom swap

/etc/fstab:
/dev/mapper/cswap1 none swap sw 0 0

Get your swap partition's UUID from ls -lah /dev/disks/by-uuid