i have build my own undergrond (for under substrate)
i used fiberglass with a safe resin
i dont know what the resin is called in english but its used on boats for building and repairs(is not bad for enviroment)
here are some pictures.

DAY1
first i cleaned my enclosement REALY WELL
then went to work
i used some chicken ironmesh to make the rough shape of the landscape
then i stuck the fiberglass and resin on







DAY 2
sand the fiberglass lightly and aply another layer of resin(no fiberglass)
before the resin dreid i drop sand on it so it sticks to it


greets Foezel