Hello and welcome to FF! Seen the excavator clay been used in dry desert type set-ups; not in humid frog vivariums. Is there a reason why you are not going with the standard carved insulation foam or canned spray expanding foam (black water proof version)? They can be shaped and covered with different substrates you mentioned, by using aquarium silicone or Titebond 3. There is lots of information on how to do those in forum vivarium section, here is one of most detailed one: http://www.frogforum.net/vivarium-te...aludarium.html .