I am definitely not an expert and am trapping in the mountains between 6,500 ft and 8,500 ft. These are some of my observations and what works for me. Yes red foxes go into cages. Sometimes they get multiple free meals.
Locations:
Riparian areas: I look for game trails along creek bottoms and set up to 50 yards above the trails, usually underneath a tree so that it is in the tree well when the snow gets deep.
Hills: I set the gullies with and without water flowing through them. It seems like foxes like to go from low point to low point opposite of cats going high point to high point.
Ranches/Farmland: I set along fence lines and next to chicken coops and sheds.
Sets:
I have tried dirt holes in the back of cages with fox gland lure and didn't have any success. What I do now is kind of like your standard bobcat set with some exceptions. Choose location and dig a hole under the pan. Work the trap into the ground and cover the bottom of cage and pan with dirt, pine needles, leaves, etc. I do not dig a hole under the back to place lure and bait. I have had foxes dig under my cage to get to it. Instead I put the bait and lure in the cage at the very back with a large rock outside the cage at the back to keep the foxes from digging at the back and force them to the front. In a tree well, I just lightly brush in the cage. Out in the open I use a piece of landscape fabric on the top and sides but leave the back open usually a couple of large rocks on the sides. I have tried heavily brushing in the cages but didn't have much success that way.
Bait & Lure:
I use chunks of beaver, bobcat, or whole ground squirrels that I catch in my yard during the summer. I also keep my dead chickens and quarter them up for bait as well.
For a lure, I use either ground up castor or Western Cats Kane killer. That is what has worked best for me. Inside the cage and up high on multiple surrounding tree.
I tested using different urines at fox sets last year with 2 cages 50 yards apart in 3 different areas. With fox urine inside the cage all I had was refusals. With bobcat urine, I had multiple catches and stolen baits. I now use cat urine in each fox set.
Other issues:
Foxes seem to step incredibly lightly inside the cages. I set the pan tension as low as I can get it, use Vaseline between the door and the release pin, and make sure that everything is moving satisfactorily once a week in my freezing conditions. Last year I had a fox steal the bait from a trap (fox prints in the snow in a direct line in and out of the trap), I re-baited without doing anything to the pan or door just a piece of beaver in the back and caught a 2 lb marten the next day. To counter the light stepping fox I use multiple pieces of bait in the back 2 corners of the cage to try and get the fox to move and step around. I will also wire bait to the back of the cage or wire a deer or elk ear smeared with lure to the back of the cage in addition to bait on the cage floor, again to get them to move around.
In my limited experience I have never caught 2 foxes in the same trap location. I always move to another nearby spot after a catch.
For me in areas I know contain foxes, I am consistently at 7-10 trap night per catch, stolen bait, or refusal (8 night average). I would probably put up my one game camera to study fox behavior going into a cage, but my average last year was $16.50 on foxes with a high of $20 and a low of $10, so not really worth it. I set for foxes in between my Marten and Cat sets, this year I only have a couple of fox sets out but have multiple catches and prints showing entry into a cage but no catch.
This trap should look familiar... this trap got robbed once a week for three weeks before the catch. Bobcat trap on a rock ledge 40 yards above this trap produced 2 cats. No fox prints up by that cat cage only down by this one. No cat prints by this fox cage only foxes, weasels, and coyotes.