The Tiffen Rota Tray with the Variable ND and the Lindsey Optics VND sound like an interesting choice for having a variable ND that goes onto a mattebox.
They are both pricey (Tiffen being a bit more)
But I have not heard any reviews or seen these in action or know anyone that owns either.
Curious to know opinions on these.

Would like to add this to the clip-on mattebox
More so interested in the Lindsey as its cheaper, but not sure