I've been using the airness harness by petego. It's a step in, which I prefer. Bama gets so excited when I put it on that the over the head styles were tricky with all of her wiggling.
I'm not noticing much in the way of matts. She has had them under her arms before
I do have a hard time finding harnesses that fit perfectly all over. It was worse when she was little, but even recently at a pet store we tried a couple of styles and sizes and only one was a good fit.
I just got the airness in a S/M because the S was getting tight. She hasn't grown or gained weight-she just got fluffier!