RANT START
Why doesn't Blue Seas (or any other vendor) have a 50A ELCI in a small dedicated flush mount panel?
They made one for the 30A ELCI (the 8100)...but not for the 50A
RANT END
Blue Seas makes a 50a ELCI that comes in a panel mounted box. You can take the the panel out of the box and flush mount the panel or you can buy just the circuit breaker and add it to a different flush mounted panel.
Here are some photos.