I know it doesn't meet your consideration of being flat. But I have stored my Denatured Alcohol in cheap 16 oz plastic water bottles. Its not my usual carry for camping and day hikes but I've done it and stored it that way. I've also used the small rounder 8 oz plastic water/apple juice bottles (again, i know you're looking at flat) b/c they are a little thicker plastic.

Normally I use a 20oz MSR bottle w/ a cap has a independent little pour spout on it that you can rotate to open without unscrewing the entire cap. It helps me identify the fuel bottle by feel in the dark or deep in my pack.