If the sharp edge problem can be solved, I think we just found the
ultimate pack-flat PSK water boiling container:
They make stainless steel
pouches for heat treating. The smallest I found was 2.5" x 5"
http://www.mcmaster.com/#catalog/115/3685/=3mnd9uVery bottom of the page.
You could fold it in half or cut it shorter. Perhaps some high-temp silicone tape over the edges.