Hey 25,
Build yourself a box long enough to hold a Victor Rat Trap. Make sure the box is tall enough to let the trap snap. Make a small opening in the front (1 1/2" hole is plenty) and you're good to go. Victor Rat Traps are cheap and work excellent for ermine.
You could also build a cubby set with just sticks and/or spruce boughs. A little ingenuity will go a long ways when trapping these guys. Just about anything that will hold a trap will work.
If you insist on using a longspring trap make sure it is set to kill the weasel, not catch it by the leg - which is illegal here.
Here is a link that may help.
www.trapperman.com/trapperman/The_Weasel_Box_Set.html