As an alternate method, as I've had zero success with glue or expoxy of any sort on
soft corals, try a hypodermic needle from a syringe. I first saw it in Coral magazine and tried it with success. The diameter of the needle is small enough that it won't leave a major wound in the coral, and it's made of stainless steel which didn't rust in the two weeks I left it on the coral. Most rock has holes small through which the small diameter of the needle will fit into.
With rubberbands, you have to be careful that the tension isn't too tight, it'll cut the coral right in two, or too loose, the coral will slip right out of it.