June: What a great idea!
Here's a good link for a sturdy card:
My name is Linda and I love the video of the outdoor card that Nan sent you. While I love the videos, sometimes they go too fast or don't really have a list of materials you need. I happened to find the exact same card with all the materials required and complete directions. I thought it might be helpful to you and your project. Wishing you the best & Merry Christmas!
Here is tutorial from the Home Depot: https://blog.homedepot.com/giant-greeting-card-christmas-yard-decoration/
Sheet Masonite, Stencils, and Exterior Paint.