Buly,
Apart from the warning noted, that sceet stuff will not provide very good airflow. I also had trouble with the coil of wire coming out at the ends. I dumped it in the end and used one of those long 3 inch foam swimming pool things, bent it to shape gradually and using duct tape to hold the shape in place, then layed up a fiberglass intake tube. Getting the foam out after cure wasn't easy, but the intake tube is strong, a perfect fit and nice & smooth on the inside.
John