Magic knot. works like a charm...might kink the coated line just a little but no damage, I do it all the time. Hard to describe but Ill do my best...
1-make a "U" with the rope where you want the length to be (hold the rope together at the top of the u so its almost like a teardrop)
2-grab the bottom of the "U" (the curve) and pull it over the top of the "u", the result will look like two "p's" mirror imaged/back to back
3-now push the two curved parts of the "p" together underneath the rope so the result is a loop that will slip knot around your finger if you pull on the two ends of the rope
4-now take that slip knot loop and make sure its big enough so you can double it up (basically you twist the loop so its a figure 8, then fold one loop/circle of the 8 over the other circle of the 8. Just like a girl doubles up a hairband or whatever
5-put that doubled up loop over the tower/tow point and pull on the ends to tighten it. Itll hold and comes out easy. Its just one big loop. Sounds complicated but its really simple once you know how to do it. Its basically a slipknot/loop thing and you double it up.
Hope that helps