HDMI video output signal has a fixed aspect ratio of 16:9, as for specifications.
When the received video signal is in 4:3 format and not in native 16:9, the box's HDMI settings allow you to automatically:
- enlarge (i.e stretch) the original 4:3 image to fill the 16:9 screen
- create a boxed 16:9 image adding 2 side black bars to the original 4:3 image
As for HDMI specifications, no other options could exist that can be applied automatically. For this reason such a plugin doesn't exist.
If you need to change the aspect ratio in some other way, you should check the available image format or zoom options of your TV set.