execute commands in a CoreOS cloud-config (e.g. to add swap) - digital-ocean

Question

I see that unlike the standard cloud-config file, there is no runcmd option in a CoreOS cloud-config file. Currently, I enable swap on a CoreOS machine by adding the following to my cloud-config:
units:
- name: swap.service
command: start
content: |
[Unit]
Description=Turn on swap
[Service]
Type=oneshot
Environment="SWAPFILE=/1GiB.swap"
RemainAfterExit=true
ExecStartPre=/usr/sbin/losetup -f ${SWAPFILE}
ExecStart=/usr/bin/sh -c "/sbin/swapon $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"
ExecStop=/usr/bin/sh -c "/sbin/swapoff $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"
ExecStopPost=/usr/bin/sh -c "/usr/sbin/losetup -d $(/usr/sbin/losetup -j ${SWAPFILE} | /usr/bin/cut -d : -f 1)"
[Install]
WantedBy=local.target
Then after initializing my CoreOS image I have to ssh into the machine and run:
sudo fallocate -l 1024m /1GiB.swap && sudo chmod 600 /1GiB.swap \
&& sudo chattr +C /1GiB.swap && sudo mkswap /1GiB.swap
sudo reboot
before swap will be enabled (e.g. as evidenced by top).
It seems like I should be able to accomplish the latter commands in the cloud-config file itself, but I'm not clear on how I can run such commands without a runmcd field in cloud-config. Perhaps this can be done either by editing my swap.service unit or perhaps by adding another unit, but I haven't figured out quite how.
So, that leaves me with two questions: (1) Can this be done or will it always be necessary to run the last commands manually? (2) If the former, then how?

Answer 1

As pointed out in this answer to an issue on Github, you end up writing a unit to invoke the command of your choice. This answer, gives a good example of using an arbitrary command:
#cloud-config
....
coreos:
units:
- name: runcmd.service
command: start
content: |
[Unit]
Description=Creates a tmp foo file
[Service]
Type=oneshot
ExecStart=/bin/sh -c "touch /tmp/foo;"

Answer 2

#cboettig - thanks to your unit file example and #philibaker note, I got this going - basically the only thing I had to do was to change the ExecStartPre to:
ExecStartPre=/bin/bash -c "\
fallocate -l 2g $SWAPFILE && \
chmod 600 $SWAPFILE && \
chattr +C $SWAPFILE && \
mkswap $SWAPFILE && \
losetup -f $SWAPFILE"
and that includes the entire setup in the preexec step.