I have found that rope and string stretch and sometimes break. Vinyl coated wire and even tough weed eater trimmer line works pretty dang well. A little something slippery on the metal perimeter sometimes helps, too, like a little dish washing soap or even some white petroleum jelly, lotion...you get the idea. Just something that wipes off easily and won't harm the vinyl or adhesive for the headliner. Having an extra pair of hands for putting pressure where the cord is can be a big plus, but after doing a few, it gets easier.