If it isn't too bad, Dermmatch works great. I still use it to this day to blend a small area above my sideburns that's a little sparse. The thin area is not something that most people would even notice, but it adds the finishing touch. And that's what a concealer should be for- to "touch up".

If you find yourself a slave to the concealers you might want to look at a larger template piece like a 3/4 or full cap. My sides were very thin and I wouldn't be wearing a piece today if it weren't for having transitioned into a full cap.