Plumber's putty was also recommended I don't think it gets as solid as an epoxy so I think you could break off if you needed to. Me? I just keep picking them up. I move them around once a week as it is while I try and figure out what the best spot is for everything so I'm not ready to commit with something that will make it hard to move. You could glue the
frags to slightly larger rocks so that they're more stable and less likely to be knocked over. I've done that with quite a few of the smaller
frags that I've gotten.