The DP and Rubies have that problem with warping. Soak 'em in hot water and reshape them, then dunk them in cold water to hold their shape. You can also use a 12" craft ring cut in half and glued in the back.
Besides getting a bigger helmet, it all depends on what type of helmet you are using. If you've got a Rubies or DP, I'd recommend using 1/2" or 3/4" aluminum stock which will make the flare completely adjustable by you. Here's what I did to my Rubies, before and after black paint.
Flat aluminum stock seems easier to glue in to me. And I forgot to add I used Devron(Devcon?) Platic Welder 2-part epoxy from Walmart to glue it in. Probably the most challenging part was to first bend the stock to somewhat resemble the inside curature of the helmet before glueing it in. Once the epoxy has set, it will be fully adjustable because the epoxy dries somewhat flexible. It's been about 1 1/2 years since I've glued this bar in and it still holds strong and is still very adjustable.