Here's how I personally think Megaton Punch would work as a Final Smash:
Essentially, you play the minigame form Kirby Super Star, complete with bar, pendulum, and crosshairs. Timed presses of the Special button change the strength of the attack. Hit the bar when it turns white, the pendulum as it reaches lowest part of it's swing, and the crosshairs as they line up to perfectly land the punch. At maximum strength, Bandana will leap (if he's in the air, he will skip this part) and then drop at high speeds to the ground. As before with my personal moveset idea, if done over a cliff or on a moving platform that is currently over a cliff, Bandana will simply fall and self-destruct. Upon landing, anyone directly hit by the punch will spontaniously fly into the background and be KO'd. Upon a direct hit, giant shockwaves will go out on either side of Bandana Dee until either a wall stops them or they reach a cliff, anyone hit by them will be sent flying, however they have chances of recovering if they are at a low enough damage due to the shockwaves being much weaker than the actual punch. If one hits a foe whilst in the air, they will be sucked into the move and dragged down with you, before being KO'd instantly. For this, however, the hitbox is rather small and one must be decently precise to land it properly.
If you miss the crosshairs entirely, the move will fail and Bandana will smack into the ground painfully on his face. Completely failing the pendulum will cause the move to weaken severely as Bandana misstimes the punch and performs it midair, decreasing the force he could've hit with. Hitting the bar while it's black (empty) will cause the move to be extremely slow and leave Bandana extremely vulnerable upon hitting the ground as he looks around with a confused expression at doing no damage.
That's just how I think of it. The crosshairs are aim and the pendulum is timing, so if the aim and/or timing is off, the move's gonna fail. And having no charged energy or anything behind doesn't leave a very powerful move.