I have used one for Image.
I found that it was not a great invention to be frank.
I got the one that has the 4 snaps that you attach to your halter.
The main issue I found was that the whole in the bottom was too small that if the grass was not just the right length it would just flatten the grass as soon as she put her head down. I made the hole a little bigger and that worked.
I gave up on the whole idea, as it was uncomfortable for her, would shift and rubbed her fur off.
I have resorted to feeding her magnesuium oxide and chromium, and lots of movement.
Its working so far.