Congrats on your possum.
Here is some very generic advice on a dirt hole for fox.
Never put lure or bait on the trap. The trap must be scent free.
Start by digging out a trap bed. About two inches into the ground. Just big enough for the set trap. Stake the trap in this bed. Put trap in the bed and make sure it is bedded solid! This is very important. If it moves the fox will dig it up. You may have to twist it back and forth to get it to sit solid and maybe even pack some dirt around the jaws and under them. After this you will have to make sure the pan is set to go off without much movement. You may want to use a pan cover such as a piece of wax paper or a coffee filter. This keeps the dirt from going under the pan and making it inoperable. Use a sifter to spread the dirt over the trap. Then take a rebar stake and make a hole in front of trap at about a 60 degree angle. The dog should be facing the hole or at a slight angle to the side. Place the lure on a grass balll and place in hole. Hole should be at least 6 inches deep. Hole should also be against some sort of backing. Like a clumpof taller grass or a small log or rock. Take the time to learn as much as you can and it will make for a more enjoyable time. It will also keep you from educating the fox in your area.
You should also prep your traps properly to make sure they go off correctly and the pan sits level when adjusted to go off with little movement. There is alot more to this than I can explain here.

Here is a site that may help. www.trapperman.com
They have a lot of info for trappers there.
Good luck.