I'm so sorry about your duck.
A fox will probably start to dig ( a friend of mine has this problem at the moment) and I strongly recommend anti digging mechanisms around coops (and even runs perhaps). Electric netting sounds good 👍
I'm so sorry for your losses 😞
I think fox is definitely a possibility, but I live in a country without coyotes, bob cats, raccoons and skunks, so I'm not experienced enough regarding possible predators.
Sadly it is very likely that the predator will be back soon. It is a great plan to put up...