Only thing I could think of is a MRE-style chemical heater (iron, magnesium, salt, and water). I never tried to heat a bottle of water with it, though. No clue how ugly that would be (since you would be putting the bottle in the boiling water). It also lets off some steam, not sure if the people around you would freak out.

That might be something to play around with at home to see if you could get away with it in a stadium.

Edit: The ones I found for sale individually include the same pouch the MREs come with. Typically you fill the pouch, wrap the top, put it in the box with the main entree and lean the box against a rock. I don't know how well that would work without the cardboard box to support it.