I have never heard of anyone catching a fox in a box trap,I think they are to smart to go in one.You would have much better luck using a leg hold trap.
There's a bunch of youtubes using large size Have-a-heart type traps...you need to cover in a tarp of large black trashbag....cover the whole thing in sticks and leaves to emulate a hole...then bait...also saw one where a guy put month old chicks in a crate placed end to end with trap covered the whole thing in a tarp with and wait....I don't trap so, I really don't know....may have to get heavier shot....