Fits just right - LaRue Tactical HandStop and IndexClip Combo

The shape of this finger-stop fits the fingers or edge of the hand just right, whether you set it up to pull against or push against. It's even comfortable if you're pushing against the back side (and you could fill the hollow on the back side with epoxy to make it feel even better if for some reason you wanted to index against the back instead of the front).
Dimensions are about 1 1/8" (30mm) wide; rises about 1 1/8" (30mm) above the pic rail when mounted; and takes up four slots (about 1 5/8" or 40mm) of a pic rail, without a clip on each end to fix it in place (more on that below). Adding a clip to each end means that the whole assembly occupies six slots.
The finger stops would add a bit of width if mounted horizontally, but if you have thin fingers like I do, you could saw, file or grind a little off the top (3/8" or 10mm) and still get good purchase.
The index clips are an unique way to fasten the hand stop in place, with pros and cons. Pro is that it uses no bolts or screws. Con is that it takes up two more grooves than it would if it bolted into place, because it requires a clip on each end (unless you're able to butt it up against something else on the rail that will serve the same function as a clip).
The clips have a great feel as rail covers. A paint-can opener works *very* well to pry off the clips. I will definitely buy another pair of these!