A 120 conibear will do fine for fisher, especially some of the newer magnum models. If you're planning on targeting only fisher, you've got a big job ahead of you. You will definitely end up with a lot of incidental catches. I'm kinda taking it for granted that you have a marten season that coincides with your fisher season. If so, all you have to do is build some marten boxes. A wooden box about 10 to 12 inches deep and large enough to fit a 120 conibear at one end. A couple of V slots is all thats needed to hold the trap in place, just move the springs up slightly for a better friction fit. Staple down a piece of wire mesh on the other end. This box is hung on a tree trunk at about chest height with the trap on the bottom, hence the friction fit. Beaver or muskrat will do fine for bait. A good lure is also important. One of my favourites is beef blood mixed with skunk essence. Sounds nasty and it is. But if you want lots of marten and fisher hanging in your shed after the season....this stuff will bring'em in. And don't forget to secure the trap to the box so that the marten/fisher hangs free without touching the tree. Fur will be cleaner that way. Hope this helps some. Send me a PM if you have more questions.