How to add usb drive as swap on Ubuntu Core using systemd

mk_usb_swap.sh

# get the usb device's id/info (for me this was /dev/sda1)
sudo blkid
# unmount it if it's currently mounted
sudo umount /dev/sda1
# make the device into swap space
##################################################################################
#### CAUTION THIS WILL ERASE EVERYTHING ON THE DRIVE       #######################
#### PLEASE ENSURE THIS IS THE CORRECT DRIVE AND THERE'S   #######################
#### NOTHING OF VALUE ON THE USB DRIVE                     #######################
##################################################################################
sudo mkswap /dev/sda1
# the device id will have likely changed - so find the new id by inspecting the current disks, looking for something like USB 
ls /dev/disk/by-id
# using the name of the device make the systemd unit for the swap
export USBDRIVESWAP=dev/disk/by-id/usb-USB_DISK_2.0_064BFA242910DCE92D9F007D-0:0-part1.swap
tee /etc/systemd/system/$(systemd-escape -p $USBDRIVESWAP) > /dev/null << EOF 
[Unit]
Description=Swap unit for USB flash drive

[Swap]
What=/$USBDRIVESWAP

[Install]
WantedBy=multi-user.target
EOF
# enable the unit
sudo systemctl enable $(systemd-escape -p $USBDRIVESWAP)
# start the unit to enable the swap
sudo systemctl start $(systemd-escape -p $USBDRIVESWAP)
# see the new memory available
cat /proc/meminfo | grep Swap