I had Keen Sights to make one for me back in the summer. They made it a custom size that fit right over the original rib and it's attached by using black silicone. I needed it to correct a POA/POI issue, and was also able to increase the perceived drop at the same time.
You can easily tell that it's an add on rib, but it cost less than $200 and made a gun that was nearly unusable for me into one that fits exactly as I wanted. Before ordering it, I made one out of wood and was able to determine the exact height I needed.
