Many moons ago, back in the Northwoods I met a fellow that made a decent setter for #14s.
I bought one as it is some kind of handy when standing on snow shoes or in knee deep snow, or when sitting in a canoe.
I would have to measure it, but first I would have to dig it out.
He took a board, probably hardwood, about 8 or 9 inches wide and maybe twice the length of a #14.
At one end, in a bit from the end, he fashioned something that the swiveled end of the trap would fit into. It may have been metal or out of wood, it was back in the late 70s.
He would slop the swivel end of the trap in place to mark where the end of the base of the trap would land. He fashioned a tapered picas of would that the base would set on. The widest end of the taper (I think the tapered piece might have been 3/4" thick, and the piece was tapered like wood shingle.
The tapered piece was fastened to the longer bottom board.
You could slide the swivel end of the trap into the holder. With the base place centered on the tapered piece (which I think was around 5" wide) and with one hand you could push down on the spring. Easy peasy, the jaws would drop open.
I think the end where the chained end or swivel end fit into the holder, the board sort of extended beyond the hold down enough so that you could put some pressure on the board to sort of stabilize it while you were using the spring down.
Reading this great explanation, it is rather hard to understand and even harder to believe that a fourth grader did not write it out.
If you are really interested in making it, send me a private message along with an email address or cell number. I will hunt that master piece of not so fine woodworking and take a picture of it. It really does work pretty well.
Mac