All the SLC eyecups collapse the same way (only the internal slot differs) so they're interchangeable. There's several mm more depth between each which is normally enough for anyone who needs it, so 10x eyecups should be sufficient on the 15x, as I found 8x are on the 10x. And of course they're adjustable, so if full extension compromises FOV you can dial them down a bit.
That said, you'd be the first here to have this issue with the 15x56. (Even I don't.) Are you sure? Do you get blackouts when the fully extended eyecups are resting comfortably around your eyes, and find yourself holding them away?