<?php
 $str = <<< EOT
#cloud-config
bootcmd:
 - echo "test" > /root/test_user_data
EOT;

echo base64_encode($str);
?>


and then


update instances set user_data="I2Nsb3VkLWNvbmZpZwpib290Y21kOgogLSBlY2hvICJ0ZXN0IiA+IC9yb290L3Rlc3RfdXNlcl9kYXRh" where uuid='bc7f3eae-2da5-4b6f-bfc2-21cdceb21b2e';


on REBOOT, the vm grabs the user_data and runs it again

I can see a file in /root/test_user_data.