I posted a youtube video of my self-made rooster collar here:
http://backyardboogi.com/news.html
I'm using a 2" wide velcro stip and it works. You can see the rooster's muffled crow in the video. He is used to the collar and it doesn't prevent him from doing all his other duties
My tips would be: Make sure it is at least 2" wide as it gives the collar the structural strength while keeping sure that the animal is not strangled. Narrow bands may collapse the air duct, which makes for a very unhappy rooster. Use industrial strength velcro as it stays snug and won't expand over time. Start loose and tighten it as he gets used to it. You can literally adjust the volume on the crowing as you tighten. Again, by using a wide band I found it to be very safe to tighten (no struggling for air - which I witnessed at first when i used a narrower rubber band).
s.