An 8oz Stainless steel hip flask is quite flat
 Yeah, that ought to about do it.  
I like plastic since it's light.  I've seen the Nalgene one that AFLM linked to at REI.  It looks decent.
Problem with a flask with alcohol is that people might think you were 
drinking the alcohol.  I can just see a cop pulling you over:  "Sure, that was for your stove, lady.  Tell it to the judge."
I bought some little squeeze bottles at REI, but they're only 2 oz each, which is fine for a cup of tea or two on a day hike, but aren't going to last long otherwise.  I can't remember what else they have, but it might be worth a visit if there's one in your area.
HJ