I bought a magnet hanger years ago. It's perfect if you have a metal door, It also has 2 pieces so you can put one piece on either side of glass.
You can try using a command book !
Beth: Command hooks make a product!
3M command hooks are a great option
Another vote for the 3M command hooks. Clean the area well before attaching.
They make Command Hooks for exterior use. That's what I use.