Rule of thumb, though--and it's what allows me to most consistently get the Illusion to up air confirm--is to hit your opponent right where Fox is going to stop since it's too difficult to really aim for any of the middle lengths during a match. Downside to that is your opponent can just hold shield and punish you afterwards, but if you're recovering then they've given you free rein to get back on-stage. It's most useful when you just randomly bring it out mid-match since jumping and buffering the side b keeps you close to the ground and gives you the lowest amount of cooldown. If you can keep the spacing for that late hitbox in mind you'll find that Illusion on-stage can put in some work, just don't throw it out predictably.
As for Fox clanking, it's pretty cool. Not sure if it's exclusive to Smash 4, but there's a specific state called "rebound" for every character (except Luma) where if two clankable attacks collide both characters' attacks are cancelled and you can see them recoil. You see it most often when two players jab at the same time and the continue mashing jab, slowly rebounding each other away. Anyway, in Smash 4 Fox's Illusion hitbox actually moves with him rather than following behind him like it does in every other title, but his hitbox--despite being able to clank--is programmed to not enter that rebound state. If a move is weak enough (nothing stronger than 12%, otherwise it will just outright hit Fox) you can use this lack of rebound in conjunction with the low endlag of Illusion when used just off the ground to put opponents in rebound and punish before they're able to act.
