Here's a method from another fly fishing BB. "Bamboozle" posted a method to hold the fly during a bow and arrow cast using a thread looped around an "O" ring.
He puts the "O" ring around his small finger. He places the fly hook around the thread and holds the tread with his thumb and index finger. To release the fly, he just lets go of the tread.
If you wear a wedding ring, you can use that by looping thread around the ring.
The credit for this method goes to Bamboozle.