Not to put words in Digital_Cowboy's mouth, but I suspect he meant wrapping the zip ties around the body of the light and then around some suitable place on the rack, to take some of the stress off the plastic mount. As you say, lights mounted on rear racks take a massive beating due to the shock from the rear wheel being transmitted directly up to the rack, due to the rack being bolted (usually) directly to the rear dropout. I have lost several this way, of various brands. I did lose PB once, but ironically, it wasn't the mount that broke, but the whole front half of the light, which holds all the electronics and batteries, came off and all I was left with was the mount and the back of the shell. Zip ties would keep the two halves together, and also spread the load out and keep the light from flexing around its attachment point. Unfortunately they will also probably block some of the light, so not a perfect solution.