Currently working on Norway cosplay, I just finished pinning my sailor hat when I saw this...
For Norway's hat, this is what I did...
1. Cut out two circles of fabric.
2. Make a hem for the hat (white fabric with two strips of ribbon), long enough to go around your head and a bit more for the trails.
3. Cut out a smaller circle onto one of the circles, as large as you need for your head to fit in.
4. Sew the hem onto this smaller circle.
5. Sew the two circles together, but with the hem on the inside.
6. Flip it inside out.
This is how mine turned out...not quite done yet, but it seems to work quite well ~~> http://satoru-13.livejournal.com/9285.html#cutid1
Norway's hat is flatter than Sealand's, I don't know if this will work with such a round puffy hat...but if you need it, I'll rewrite it with more detail on the measurements and sides of fabric and such.
Hope I helped!