your first idea is the more conventional method. It will adhere perfectly to the contours of the space if you use good technique in laying the fiberglass down. This means not trying to cover the whole thing in one sheet, cutting/tearing the fabric to better conform to the contours, as well as using a roller to consolidate the layers and press out any air bubbles. This also improves saturation.
the second idea is intriguing. i would suggest going to a packing store and getting one of those bags with the expanding foam inside it. this might give you a more rigid structure to work with and contain your mess. Keep us posted on what you decide!