I planned to insert pieces of fusible web and iron them on.
After riping out a seam at the bottom of the hat I found it impossible.
Fortunately, I had one more solution prepared. Fusible web didn't have to be sad as it was in the plan as well.
I cut three ovals from the web and pinned them inside each horn. It wasn't easy and in the end it wasn't perfect but somehow I pinned them.
Now my surprise - glue gun. I bought one. And I managed to spoil it. Now it's messed with melted glue and doesn't move. Skillful hands..
I bought a smaller model. I was more succesful with it. I couldn't make a continuous ring of glue but I put each pin aside and glued the spot under it.
It looked like pinned with ivisible pins in the end:-D