Tener Aeon

No simple answer, so much depends on the Hat and your hair. If hat and hair is copy and mod, you can do all sorts of things like size the hat, attach the prims from the hair that you need to it, etc. If the hat and or the hair is no mod, you are kind of limited to finding a hair that will work. If they are no copy, remember that anything you do changes the only one you have, so be careful. I usually just resize the hat to work with a hair I like. Not always the best, but works 90% of the time unless the hair style is really poofy. Hair is never going to not show through the hat from time to time if there are flexi prims in it. When you walk, run, dance, etc, the hair is going to move through the hat, just a part of SL most of us ignore, just like hair sticking through the middle of us when we dance with long hair.