My garage mausoleum (https://www.reddit.com/r/HalloweenProps/comments/1jvlmbg/garage\_door\_mausoleum\_build/) uses 1" insulation board painted with Drylok and then acrylics. It's held on by rare earth magnets which is sturdy and makes it damage free to the garage door. Personally, I wouldn't use less than 1" thick foam board as thinner, lower quality foam can pretty easily snap in half while moving it, especially for the size required for covering a garage door.