I believe the only way would be to configure your camera stream for a different resolution/aspect ratio – one that better matches the Alexa device/FireTV.
Using FireTV devices on my HD TVs results in a default aspect ratio of 16x9. So if I configured my camera stream (via the camera settings on the camera – not via monocle) to use a resolution of 1080P (1920*1080) I get a full screen view of the camera with no black bars.
Thanks, Robert