We deployed a tweak for this a while ago, which mount binds /tmp to /mnt/tmp so you can take advantage of the ephemeral storage.

The tweak is applied to all newly deployed servers, but if you need to apply it manually to a running server then it should be something like this:

mkdir -p /mnt/tmp
chmod 777 /mnt/tmp/
chmod o+t /mnt/tmp/
cp -ar /tmp/* /mnt/tmp/ >/dev/null 2>&1 || true
mount --bind /mnt/tmp /tmp

Ephemeral storage should survive a reboot, but not a start-stop (ebs backed).

If you would like the mount binding to happen automatically on reboot / start-stop, the following should help (note that you don't need to do this on newly launched instances:

FIRSTBOOT_MNTTMP=/usr/lib/inithooks/firstboot.d/26ec2-mnttmp
EVERYBOOT_MNTTMP=/usr/lib/inithooks/everyboot.d/26ec2-mnttmp

cat > $FIRSTBOOT_MNTTMP <<EOF
#!/bin/bash -e
# mount bind /mnt/tmp (ephemeral storage) on /tmp

is_mounted() {
    mount | grep -q "^\$1 on \$2 "
    return \$?
}

# skip if already mounted
is_mounted /mnt/tmp /tmp && exit 0

mkdir -p /mnt/tmp
chmod 777 /mnt/tmp/
chmod o+t /mnt/tmp/
cp -ar /tmp/* /mnt/tmp/ >/dev/null 2>&1 || true
mount --bind /mnt/tmp /tmp
EOF

chmod +x $FIRSTBOOT_MNTTMP
cp $FIRSTBOOT_MNTTMP $EVERYBOOT_MNTTMP

I hope the above helps...