[Updated 5/7/16] There are 4 printed parts in all, Side Spool A and B (or Side Spool All), Side Guide Switch Bolton, and the tension nut. Print in ABS or PLA, 25-35% infill, 5 @ M3 x 10-15mm machine screws, 6" section of 3/4" Sch 40 PVC pipe, and a tie wrap. You may need to ream out the guide hole with a 5/64" or 3/32" drill bit to clean it up after printing. The limit switch arm needs to be rotated in Y by 270 degrees to lay flat on the bed (see CAD rendering).