The Frame Mask plug-in adds a framed mask around an object (e.g. image or geometry). In the properties, the aspect ratio can also be set.

Note: This plug-in is located in: Plugins -> Shader -> Effects

Frame Mask Properties

Left, Right, Top and Bottom Edge: Sets the tracked borders (see also Tracking).
Left, Right, Top and Bottom Gradient: Sets the gradient from the edge of the frame to the edge of the object.
Aspect Ratio: Sets the aspect ratio (16:9, 4:3 or Custom), if required (default is Off).
Custom: Uses to set a custom aspect ratio (e.g. 1.85:1 or 2.39:1).
Tracking: Uses a Container to track the location of the actual frame mask instead of animating all tracked borders manually. Drag a Container to the drop zone (1):

Plug-ins User Guide