I would use a flat resist. The resist should be shaped in a rectangle (larger than you want your diameter to be by how much shrinkage you've calculated). Then put your wool on both sides of the resist and covering one end. Felt until hanging together, then remove flat resist, open into the tube shape and felt/full into shape. Have you used a flat resist before? I am sure there are more posts on using a resist here as well. I use "fun foam" - a product for children's crafts. Other people use bubble wrap, pool covers, wrapped cardboard. Anything flat that will hold up to the wetness and that you can cut into your preferred shape.