Most of our doors are held open to prevent a cat or a Roomba from getting trapped. Bungee cords work well, but there isn’t always a place to attach them. 
 

This is a thin, 2mm, sheet with a loop on one end.  The sheet slides nicely under the frame of a near-by door allowing you to use the loop to hold onto a hood – in this case the hook of a bungee cord.  If 2mm isn't quite the right thickness for you, you could scale it up or down a little.

I used organic supports to help print the loop.  Nothing special about the rest of the model.