I bring the line through the hole in the control horn twice, this helps to cinch the thread so you can do a precise tension adjustment, then I just tie off with half hitches. A whippy boom would be trouble to rig if your control horns are positioned away from the center line, but if you can keep the control horns closer in, there won't be much leverage working against the boom. A CAP 232 type tail arrangement probably works best for this idea because there is no conflict between the rudder and elevator hinge lines, so the elevator control horn can be placed right next to the fuselage. Dacron is too stretchy, I think you would be constantly fiddling with it. Monofilament might be a happy medium between dacron and spiderline.