The epoxy that is reef safe and you slice it off and work in your fingers.. but with mixed results. It only holds shape and needs something to grasp after it hardens. .. and it still isn't all that strong of a grip.
You can use zip ties, or plastic pins too. Most LR has enough holes you may be able to find a place or two to poke zip ties through..
....or can drill one out better.. but this frequently results in a new piece of LR rubble.
Mine is just carefully stacked by looking for combinations that look good, kinda lock together, and start with a good base solid on the glass and not on the sand. I'm a retired machine design engineer and thought I would figure out a way to fasten it whcih was secure and easy.. ... .. not.. I gave up because everything I tried tended to break up the rock.. and make a huge mess of the project. My take, for me, stop killing the live rock and stack the darned stuff, intelligently.