I‘d imagine a roller block is designed for a certain diameter ball. A larger ball will have a different curvature than the cups in which it sits. And that can impede its movement.
There is a limit of course to the ball size that is constrained by the "cup".
But you don't need to have any curvature at all - just a lip of sorts to contain the movement of the ball.
In fact, I would think that a curved cup adds resistance to the free movement of the "roller".
A flat cup with a lip will allow use of a larger roller and have better freedom of movement.
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