I like fox but are a bit dissapointed with his smash attacks. Side smash has a bit too much lag for me and feels predictable and up smash needs to get close to finish off. I want to better at finishing
Only answer I can give you is: You have to bait them.
In order to get better at finishing in general, you have to study your opponents' habits and look for when they're open.
One of the ways you can do this is by walking around your opponent when they're at kill percent. Fox has a fast walk speed to help him out.
Walking around your opponent can cause hesitation which can lead your opponent to react differently depending on who you're facing.
Against Fox, some players like to hide in their shields, spot dodge, roll or try to knock you away with an attack just because of the fear of that up smash and it's up to you to react and punish these options accordingly.
Also don't forget Fox's bair. That move has good kill potential and is fairly safe on shield (well at least in comparison to his other moves.)