I've had the Yakima tracks on two different Retrax bed covers, now with a set on my GMC Canyon's Retrax Pro. If you plan on off-roading, make sure to use all of the mounting points. On the Canyon, there is just enough room to mount with SS button head screws, with washers top and bottom, and clear the bed rails underneath. I turned my cover over after mounting the tracks, and flattened the tips of the machine screws and nuts with an angle grinder for better clearance, also locking the nuts on very well. I just returned from four days of very rocky and washboard roads in Death Valley, and like with my previous Nissan Frontier's Retrax, the mounting is un-fazed by the torture.
I also recommend using the square track nuts available from Yakima if you plan similar roads and trails. I did distort the tracks some on the Nissan using the standard Yakima jam nuts, since they don't provide as must surface area inside the track, and after changing out to the square nuts had no more problems. These square nuts need to be installed with the track and left there permanently. I've included a photo below.
I took these track photos on my recent trip for someone else, so good timing on this thread. The bars are custom, originally made for a roof-top tent on my Nissan, and with minor modification adapted them to the GMC. Now I carry a Rhino Rack Pioneer Platform on the load bars.