A while back I saw a street-style image in which a guy was wearing a brimmed hat that looked to have been made bigger by slicing and then loosely safety pinning the seam. So when I found this black Bates Hatter hat at a thrift store I wasn't too upset when it didn't fit on my dome.
I should have taken DIY process images for you guys because it was really simple to do. I used a sharp razor blade to slice up the back of the hat and then used my leather punch to make even holes for the cord to lace through.
Voila! A quick hat expansion.