your drawn idea looks pretty good. is this eye cover needed for protection, or just to cover an unsightly socket? you said the lid or skin was sewn shut, so i can't imagine it's needed for protection. if the owner is just wanting something for looks, then it should be as small and light as possible, and made to blend with the bridle. your drawing looks like you've added a cavason to attach the bottom of the cover, that could certainly be made to match the bridle. either pad it good or make sure that it doesn't rub over the socket.
or maybe the owner would like to be creative and have a flashy pirate horse!
i'm interested to see how it turns out. can you share pictures of the finishes product?
my wife was always customizing equipment whenever one of our horses were injured. but we never had one lose an eye.
good luck!
ken