I've been using 3 mm Spectra that I get from Masahiro Mori,
Morifish@aol.com. I just tie a simple overhand knot in each end and insert it in the rubber using a wishbone insertion tool, use a constrictor knot with waxed whipping twine to keep it in there, and finish off with a piece of heat shrink to protect the whipping twine from abrasion. Its not entirely necessary to taper the ends of the bands, but if you want to do so, stick a nail in the end of the band to give it stiffness and hold it against the side of a grinding wheel and turn it until you get sparks from the head of the nail.
The wishbone insertion tool is that stainless tube with a slot in it sticking out of the vise. I should have cut the long end of the cord off to the left of the knot before taking the photo, but anyway it greatly simplifies getting the knot into the end of the band. Just spray a bit of silicone on the knot and into the end of the band and then push the band over the knot in the end of the tool.
