I think the key in this matchup is to not get yourself caught in your shield in the first place.
Personally, I like using many Nairs (to rejumped Nairs) since it helps slowing Fox down and eventually land a grab or DSmash. Jabs are also good at stopping approaches, Uair is also good against "aerial" approaches and can lead to combos.
I believe Dash Attack an Dsmash are the best moves to juggle Fox at lowish percentages (< 35), DashAttack is really fast and can catch the Fox off while he trys to DI to the sides (after a Dsmash f.ex.) .
Then you can start adding Utilt and Uair for aerial juggles.
At least, that's how I proceed...
By the way, which one of these moves is faster? Nair OoS or Up-B OoS? I still can't quite figure out which of these is the fastest, even with all the frame data I've gotten lately. I still believe Nair is the faster move since the hitbox comes out a few frames first but meh... someone please enlighten me ^^"