This is a printable line cutter with 2 versions of holster (one for placing onto 2" belt another for tying/hanging using 1/8 shock cord).

The cutter is designed to be used with 1/2"x0.025" (its about 12x0.65mm) handsaw blades. the blade length doesn't matter as long as you can break off a piece about 2" long.
Sharpen the blade on one side. Break it to fit inside the cutter (it goes back up to the end of the view port on the side of the cutter) and grind to round the ends a bit.

Saw through the perimeter in the middle of the nose of the cutter (see the picture) and push the sharpened piece of the blade through the cut. It is a tight fit. You will have to work it through. The plastic of the perimeter where the blade has to stick will have to come out (might use another tool or try to push it out with the blade itself).

I've used a trashed handsaw blade that is not corrosion resistant. The stainless steel blade they sell for butcher handsaws is probably a better choice for the cutter blade.

Print in ABS. You might try to play with infill perimeter overlap in skeinforge when printing the cutter since it has to bridge the opening for the blade and in several places the bridge hangs on just a single perimeter wide wall. It's not a big deal if it fails in a few places on the first pass (it will recover on the second and all that mess will stay inside). If using a big nozzle make sure slicer does not remove that thin perimeter wall (it's about 0.6mm thick).