I actually have the same setup with my Garmin Echomap, but elected to not use at area to mount my FF because I like using a ram mount and it already had a track mount Just to the right side on the rail of the kayak.
I also got way more used to having my FF within arm's reach on my outback, and at a much better angle for the sun and my shadow.
I just use that hatch to store my mask/snorkel and weight belt when I dive
I'd recommend something akin to a kickstand, maybe a hinged item you could screw into the cover?