Any time I frag leathers, I use the toothpick method to mount them. I use a piece of live rock rubble, and glue a toothpick to it. Better to actually insert the toothpick into a little hole, so feel free to drill one in the live rock. This will give it more structure, as a simple glued toothpick to the surface of the rock will obviously detach in no time. Cut the toothpick down so that after you insert it into the rock, you will have enough sticking out that you will get good penetration, but not so much that it will come through the frag. It helps if the rock surface is nice and flat around the toothpick. I don't use CA or string, just take the frag, stick it on the toothpick.
I have an acrylic box made, with a bunch of flow holes that still allow water movement. You want to put it in an area where it still gets flow, but not so strong that it will take it off the toothpick. In all my cases, the leathers have attached to the rock with 2 weeks. By attach I mean fully, not just a little piece.