I wanted to incorporate a spring as the other designs I've seen rely on require getting the "timing" between the two nuts perfect (which I was unable to do). I sort of copied http://www.thingiverse.com/thing:151011 but that was for the aluminum carrier. Also I have the short lead screw, so I mounted it underneath. I had to replace the screws with some long 4/40s. I used a brass nut for fun and the spring I found was a Servalite #131U (1-1/16"L, 5/8"OD, 0.047 Wire).