I have a Yakima gatekeeper pad that is really nice, but don't use it for solo, local trips. When I'm just going across town, I just fold up a beach towel, put it over the tailgate to prevent tire rub, and put a cut piece of that denser pipe insulation on the frame, where it makes contact. Then use a bungee strap over the frame at the seat tube/top tube junction. Rear wheel up against the wheel well. Works just fine, nothing moves, and is way quicker.
With my alum bike, I would just lay it over in the bed, with a bungee snaked through it, just in case. Also worked fine, but I'm not doing that to my carbon baby, even though the frame doesn't actually touch. Just bar end, pedal, and tires, but still....