This can be very difficult to do with a frag on a bit of Live rock.
Here's a method you can employ to make life easier and safer for the frags to ship.
You will require a piece of styrofoam.
Cut an X into the piece and insert the Coral Cradle into the X. Done!
If you have a plastic bag that was round like above's picture, you could make your styrofoam round, about the same size as the opening of the bag to give it a snugger fit.