It's pretty easy to run an instance for a few minutes while you rsync the whole thing wherever you want. We used AMIs mostly because we ran the CTF itself in AWS and didn't have a great place to store the images otherwise.
Thanks for putting this on and leading me to smashthestack.org, I had looked (admittedly, a shallow look) for a CTF style game after doing one at a conference last year. These things are crazy addictive to me..