What a fun idea! You can use these hats for adults, it won’t work quite as well, though. While the hat will probably fit on top of your head, the sides and back won’t come down far enough to cover your actual hair, and that makes it less fun. I googled and amazoned some other hats, and didn’t see any that would work for covering the sides and back… so the next best option would probably be an adult-size beanie. If you search “skull cap beanie” you’ll find some with a good shape for covering all your hair. You should still be able to make it with a hot glue gun, I would get a foam head from the craft store to put the beanie on while gluing on the feather boas.

You can find them under “felt hat” at Consumer Crafts: https://www.consumercrafts.com/store/search?s=felt%20hats&p=1&ps=20

I sent you an e-mail earlier, but thought I’d respond here as well in case someone else has a problem finding the hats. The simplest place I’ve found them online is ConsumerCrafts.com: https://www.consumercrafts.com/store/search?s=felt%20hats&p=1&ps=20