The vinyl cover has a foam-like padding on the back with a smooth surface. The edge or lip attaches to a groove in the dash pad. The airbag lid itself is a hard plastic of some kind.
The helpful hardware man knew a lot about glues, and recommended E6000 Industrial Automotive adhesive. Grey tube.
So I tried it. Parked facing south on a sunny day. Dash pad and flap got very soft and flexible. Roughed up the hard plastic with sandpaper, cleaned things off. Adhesive is thick, but will self-level and run downhill. It acts a bit like industrial strength contact cement. I worked fast with a popsicle stick and a flexible putty knife. Says it starts to set up in 10 minutes, but cures over a day or more.
I got some tube sand since it's skinny and long, heavy, and moldable. Let out some of the sand as it just wouldn't fit tight to the windshield full. Once the glue was spread everywhere (use acetone to clean it off of things), I wedged the sandbag in up against the glass and pounded it into the shape to fit tight.
No idea if this will work. I'm going to leave it sit for a day or so.