This is a duckbill scriber for marking chairlegs and scribing lines parallel to a surface. The hole for the pencil is .25" diameter, I think a number 2 pencil is .25 across the flats, so I think..I think it will interference fit in the hole. If not maybe you can use a wedge of sorts. Feel free to comment and maybe I'll revise it accordingly.
Okay, I added a 3rd part with a slice in the pencil holder so that you could use a 10-24 clamp on it. I used the thread part in fusion 360, so I have no idea how that will print out in 3d.
Update 10-24-17 - I revised the center section (REV3) file. The hole is now sized at .3" diameter. I also included a 4th revision that has the hole sized at 8mm or .315"
In addition to resizing the hole, I have widened the slot on the bottom to mesh better with the outside portion.