I have a few montipora digis and an acro in my tank. I mounted them by using Viper's first suggestion; sticking them in a crevice and allowing them to encrust.
Originally posted by RUBBERDUCK
what exactly do u mean by encrust?
They grow onto the rock and 'encrust' over it to get a good solid foundation before they start to grow outwards more.
Just use super glue.It won't harm them and if you have any RTNing at the base cover it all up with the glue it should keep it from advancing on up the coral.