I wanted to keep my Ruuvitags on top of a molding on half paneled wall, but they kept falling due to slight angle on the molding. So I designed this mount to slide a thin spline between the molding and wallpaper keeping the Tag from falling.

I included FreeCAD file to customize the spline thickness so it can be used with any gap between wall and some other object. The file also allows you to customize the shelf rotation so it can be slid under a vertical gap or even gap in a ceiling. Just modify the values in settings spreadsheet.