There are two very good Foxes in our group including our #1 player, and I've taken a few games off each of them like this.
Don't camp; Be aggressive. Fox will depend on rushing you down, so you have to outrush him. Look for openings to grab, b-reverse egg lay... Nair is your friend, but be very careful coming in from above. Nair will trade with most of Fox's attacks, but he'll almost always hit harder - especially his Usmash. It will, however, interrupt a lot of his combos (but don't forget you have super-armor on your djump! That's often effective at getting you out of a combo where Nair isn't cutting it). Try not to jump into him with Nair as an approach, but if you knock him back, tech chase him and have a Nair ready for his getup. The more you can keep him off his feet, the better off you'll be.
Once you take the aggression, he'll probably start laser camping to bait out an aerial approach. This is where it gets tricky, because even our typically safe retreating-Fairs aren't that safe as Fox can often DACUS fast enough to punish that. This is where B-reverse egg-lays become your best asset, but remember to mix it up - if you do that every time, they'll just pivot Usmash or catch you with Utilt (it starts behind Fox), so occasionally land early and run in for a grab.
Remember that Fox is a fast-faller. Abuse this. This is where my favorite trick comes in handy, particularly if he's laser-locking; Grab him as close to ledge as possible and chew until release. If he stays grounded, jab-jab or dtilt him off stage. At high enough percent, he'll have to respond with Fox Fire to recover because you'll knock him low - if he's away from stage, he's a sitting duck for a Fair spike. If he gets close to stage, then stage-spike him with a Bair. If you land it early, he could dj into range to Illusion his way to ledge. A lot of times, they'll gravitate towards their max distance - you can either predict a punish at this position (Fair spike comes to mind again), or pre-empt a ledge trump.
Finally, ground pound if they get shieldy. Get some shield damage in there and go for the shield break. Remember though that this is a very punishable move, and Fox is great at getting into any gaps and dealing a lot of damage. Wait for the right moment - save it for when he's on the ropes and you notice his shield is weak.