M.6.
Having written numerous letters for appealing insurance decisions (and luckily have only lost one), I have found that it is important to include any other treatment or therapeutic measures that you have tried that have not worked, make sure that the Dr. or PT person includes the deviation from normal that the asymmetry is, the projected benefits of using the helmet instead of continuing with other methods, and if there are any potential cost savings benefits to getting the helmet.
My SIL/BIL twins needed helmets due to prenatal positioning, but their insurance did consider it to be "cosmetic" so they helmets were denied and they had to pay for them themselves (but did max out flex dollars that year so that helped).
Good luck!