Sorry for the wall of text, but I do play against a Fox main every weekend so I know this matchup better than my own main

So... Yeah. I think this is an important one so I'll give you everything I know on it.
Rather than exploiting strengths and weaknesses, this MU is all about being aware of what Fox is looking for.
Fox has a some things for you to look out for, especially as Lucario:
1.) Fox's Utilt. That thing is scary. Be careful at low %, because you can get walked into it quickly. You generally want to try DIing on the side Fox is facing and try to jump out if you think you'll be safe.
2.) Fair spike. Fox mains might do this thing where they fast fall their forward air offstage, pulling you down into a footstool. Be really patient offstage (especially since your recovery is pretty good) and you should be fine. Fox normally has trouble edge guarding and especially if you're not Cloud or aren't between him and the stage. If you are, get ready to tech some Bairs
(Edit: if you get thrown when not offstage, DI away from Fox if they like using Fair. If not he might get a free combo. Be prepared to tech or avoid a dash attack too. If you're offstage, moving away from the stage is your best bet)
3.) Uair. Fox's Uair will be used a lot in combos and will probably not be punishable, especially when autocanceled. Fox is always looking for this, Bair, or Usmash at high %, so get ready to DI left or right if you're hit by Uair or Usmash at high %. Also watch out; Utilt can true combo at certain times but usually 50-50s into this.
4.) Usmash. Fox has KO setups out of Dair, Nair, perhaps fast fall Fair, and jab 1-2. At high %, be ready to tech and mix up the roll. If you don't tech, you WILL get Usmashed and probably lose an early stock, watch out.
5.) On KOs and stages: Fox has trouble KOing, especially when you deny him his setups. Lucario is heavy, allowing for Fox to keep his KO setups longer, but has aura, so that helps. Also, Fox loves Battlefield, Town and City, and stages with low ceilings OR platforms. Go somewhere where you should survive the longest, especially off the top, and let aura do its thing. As for platforms, Lucario's heavy weight opens up even more options to Fox's already amazing combo game, so getting caught under a bunch of Uairs where Fox is shielded by platforms is not a very pleasant situation.
6.) On neutral: Unfortunately, this is the biggest part of the matchup imo, and it's one of the worst parts for Lucario.
That being said, I'll admit that it was a really disgusting matchup for me, especially because of neutral. Fox is really fast, has really low lag, has superior priority, blaster for free damage/approaches and reflectors for aura sphere. Punishing Fox, especially when you're on the ground, is really difficult. However, if he's above you, Uair can help... but be careful. If he lands a hit, it could mean a combo or a stock. You're never really safe from Fox, even when he's landing.
Edit: There's people like me out there who enjoy a flashy Aura Sphere snipe, but I don't enjoy getting a max aura AS reflected back at me. Odds are Fox mains are aware of this and will have out reflectors in the late game. Also watch for Fsmash; it's safer than one might expect even if it doesn't move past your shield.
Fox's dash attack also can set up into an Uair at certain points but I'm pretty sure it's not a true combo.
Hope I didn't forget anything
All in all, I never enjoyed this matchup, so I can tell you from experience it may get frustrating. Hope this helps, and my apologies to all of the people who have to scroll through these 5 miles of text
