I quit making my own marten boxes when I realized how bulky and heavy they are, even if you haul them on a sled. I went to flower pots and motor route tubes. They're light and a bunch will fit inside each other. I'm not trapping anymore, but here is how I used to do it.
Flower pot set: Find some plastic flowerpots, solid ones, not flimsy, big enough to stick a 120 bodygrip across the opening. Cut two slots, opposite each other, in the rim of the pot, about an inch wide and approximately 1.5 inches deep. This is where your 120 springs will go when the trap is set.
At a ninety-degree angle from a line between the slots, cut two holes in the side at the top of the pot, near the rim, and two holes near the bottom. You will slip annealed trapping wire through these holes to wire the pot to a tree, hanging upside down. In the bottom (now top) of the pot should be two more small holes for wiring your bait inside the pot. I also liked to use a visual attractor, like a bunch of grouse feathers or a strip of fur, beside the bait. Also, a visual attractor that will move in the wind is a good thing to hang outside the set but nearby.
The marten sees the attractor, finds the pot, and gets nailed when he tries to grab the bait. I'd always hang the trap so the marten hung above the ground. Lots of carnivorous shrews out there.
Motor route tube set: These are the boxes you see hanging below mailboxes for newspaper deliveries. They are used exactly like marten boxes. I believe I got mine here:
http://www.continentalproducts.com/motor_tubes.php I found that I liked the extra large size better than the regular, mainly because they have a larger opening and will hold a Belisle.
I would bait and wire-up a bunch of these pots and tubes ahead of time, wind the wire around the tube or pot, and stuff them inside each other. When I got to a good set location (usually where I saw marten tracks in the snow), I'd whip out a pot or tube, wire it up, and be on my way. And only have to take my gloves off for half a minute of wiring.
Jim