Grinner and Mr. Jameson mentioned 3 of the best spots to find a Ermine. Stone walls or piles of rocks. Bruch piles. Cattail swamps. Those 3 are killer spots, always have Ermine around.
You have to find the locations. They will roam all over but hunt in specific areas. Go to those areas they hunt.
Bridges, are there rock piles under or near the bridge? Set it on BOTH side of the bridge. In the rocks, put a rock on top of the box, paint some boxes dark grey, they will blend right in, put them at the waters edge and right next to the bridge wall. Both sides, so two traps on each side, one next to the wall and one down in the rocks at the water. Go to the other side of the bridge and do the same thing, that's 4 traps under one bridge.
Any old abandoned buildings? Set inside and outside along the walls.
Any old machinery sitting in the weeds? Set a box under it.
Any old places they dump anything? Set it, right in the middle of the mess, try to get the box under something, a old car hood or something dry.
Use lot's of bait that the mice can get to and a lure up a couple of feet over the box. Smear it right on the rocks or bridge wall.
Your cat's only wish they could catch an Ermine, they can, but not easily or often. Hide the traps from the cats view.
Set that barn hard, and they will be in those culverts hunting and using them to access other areas and stay off the road. Cover your boxes from your cats view. Makes the ermine more comfortable to work your trap.
Maybe you have those big round bales of hay. I would set a box in there for sure. Think of all the mice and small creatures living in those bales! Good food and a nice house for a Ermine.
I am in Alaska, and these are the places that produce Ermine every year, all season. Find something similar on your land and go after them. Rat traps, #0 longsprings & jumps and #1 coils, longsprings or jumps. Holler if you have questions. Mr. Jameson sell really good Ermine lure too! Check his website out. His Erminator is a killer lure.
https://www.jamesonsultrablend.com/