You can create a collar using data frame or map frame within the layout view, as follows
Set the data map position for both X and Y to 0 (Zero)
Increase the thickness of the frame to desired value (I used 20 mm), The thickness increased from the center of the line (both outside and inside of the line), that is why I set the X and Y position to 0 to exclude the outside from being shown when exporting the map to an image or a PDF.
Choose the desired color, and set the opacity to 50% or 60%.
The result is shown below:
If you want to exclude the yellow color in the right side of the frame you can add a rectangle to hide that color and lock the layer to become a space to put some legend and other information, and the result will be like this:
The output when you save the layout as image is like this:
Also, there is another option to create a collar using rectangle object, but a lens-like shape, using the following steps:
- Create a rectangle shape that covers map frame completely. The extent of the rectangle is same as the extent of the map frame.
- Under Items -> select the new rectangle -> Style and click on Change
- Change the style from simple fill to shapeburst fill, and change the settings as shown below:
- You will get an output a focus lense-like shape, as shown below:
Using also @RyanDalton's method, it is better to change the transparency of the rectangle (collar) itself rather than the map frame to keep the map frame very clear. Also, changing the "Blend mode" to 'Soft light' OR 'Overlay' can some effects, but it is still a personal preference, as shown below:
(1) Blending mode: Soft light
(2) Blending mode: Overlay with 25% transparency