Appoligize for having my ugly mug in the photo, but here is a Kury type rack with the star washers.

As stated above, use a rubber washer under the star. After drilling I placed a piece of duct tape on the inner lining and used a small amount of window/door type foam fill insulation to go in the dead space between the layers.
Not expanding foam!. Once it sets up , redrill the hole and you're ready for the install. I also used rubber washers on the inside with a large washer similar in size to the star washers.
Don't over tighten!