A gland type lure will work, my favorite is Minnesota Trapline Products "gusto". It's a strong, skunky lure generally designed for fisher and marten trapping but I've found it really gets a canine's attention also. Use very sparingly, a tiny dab goes a long ways. Great for mean spirited practical jokes, DONT get any on you!
A bigger problem you might be having is that canines have a natural aversion to entering enclosed spaces other than natural burrows. Even those they are pretty skittish about. They can be trapped in box traps but it's much more difficult than a dirt hole or flat set with a double staked # 1 1/2 or 2 coilspring foothold.
I'd just use a shotgun with #4 shot. When fired at ground level that won't carry more than 100yards max. Plenty of power to dump a fox.
For your box trap I'd set it along a wall or brushpile or some natural edge where the fox is already moving. Camouflage the trap with loose grass, brush or something. Be careful not to get any of your scent on it. Wear rubber boots and gloves.
