I'm working on a piano part where the player needs to slam their fingers on the keyboard in semi-specific regions to create random cluster-like chords. What do you think would be the best way to notate it? Should I write the exact chords played in the MIDI recording I'm transcribing (maybe with X noteheads)? Or should it be like the standard cluster notation (MuseScore's notation is bad and hardly implemented, this is only a mockup):
The top and bottom notes - which define the extent of the cluster - are usually shown more distinctly than in your mock-up. Black-note clusters are shown by a sharp or flat-sign; white-note ones by a natural-sign.
You mention 'semi-specific regions'. You could simply show the approximate register and approximate extent of each cluster and write "A piacere" or "libero".
But whether the notes are black or white makes a huge difference and should surely be indicated. You wouldn't want the performer banging away on just black notes: it would sound like a child playing.
Mixtures of black and white are simple: one hand or arm plays the white notes and the other the black notes.
Have you looked at Henry Cowell's music? This page has some useful stuff about his cluster-notation.
If it's for live performance remember to wear long sleeves. Some pianists wear gloves. For larger clusters you may need lengths of wood, which should be wrapped with something soft to protect the keys.