The only collage quilt I've done was a Laura Heine. I took a class from her and she didn't use tulle over the top. I haven't finished mine yet (its only been 3 years!!!!) but as I remember she used steam-a-seam to place and secure the pieces on a light weight interfacing (I think) foundation with the design drawn on it and then glued it to the background and finished by straight line quilting. You might look up her technique. It was easy and fast.
You have an amazing start on your rooster. I can hardly wait to see the finished product.