For a more precise fit (and a really easy one at that, that doesn’t require sewing!), follow these instructions:
You need: your hat, scissors, double-sided tape and some foam padding material. Try to find padding that is no bigger than 1cm thick and about 5-8 inches long. The bigger the hat, the longer you’ll need the padding to be.
Measure how wide the sweatband on the inside of the hat is. Cut your foam strip a little more narrow than the band. Before taping it in, just try it out and tuck it behind the sweatband and put the hat on to see the fit. If the hat is really big, you can put two strips of foam on top of each other at the back (never at the front).
Apply double sided tape to just one side of the foam strip.
Take your hat and peel back the sweatband at the back of the hat.
Apply your the foam strip with the sticky side down to the inside of your hat. Press down on the foam strip to secure it.
Fold the sweatband back up and if you measured it right, it should be invisible behind the band.